Temperatures will fall to as low as -9C this morning as Britain's cold snap continues, after planes were grounded at Heathrow by the Arctic blast yesterday.

Freezing fog will stay in place once again in some areas today, the Met Office said, while the mercury will hover just above minus 10C in parts of southern England. 

The UK Health Security Agency (UKHSA) extended a level 3 cold weather alert until 9am on Friday, with people urged to keep warm and check in on family and friends who may be more vulnerable. 

As temperatures plunged to minus 9.5C in Santon Downham, Suffolk, on Sunday night, it was confirmed hundreds of households in England and Wales are eligible for cold weather payments. 

They are made to vulnerable people, including pensioners, to help them pay for heating when the temperature dips below freezing. 

The weather alert is described by the Met Office as 'severe' - meaning that people should take precautions and stay tuned into the UK forecast for potential updates. 

It goes to those living in an area where the average temperature is recorded as, or forecast to be, 0C or below over seven consecutive days.  

Payments will be made to homes across north-east England, Cumbria, west Wales and Oxfordshire. 

Meanwhile, the Environment Agency has 34 flood warnings in place across England, meaning flooding is expected and action to prevent it should take place. 

Met Office forecaster Craig Snell said: 'We've got rather cloudy, damp but mild weather across Northern Ireland, Scotland and far north-west areas of England on Tuesday, temperatures could get up to 12C or 13C across Aberdeenshire way. 

'Further south across the rest of England and Wales we're still under the colder conditions, so there could be some fog again which will be quite slow to clear, so a very similar start to Monday. 

'When there is cloud it's going to feel pretty cold, but in the sunshine, although it will be cold, the sun will help negate the cold a little bit, so all in all a pretty similar day to Monday. 

'Going into Wednesday we do see a change as the day goes on, we've got a cold front coming down from the north so that will move across Scotland and Northern Ireland during the morning and then gradually move across England and Wales during the afternoon. 

'There will be some rain with it, turning brighter after it moves in, and the front will then pass on Thursday.' 

He added: 'Tuesday morning will probably be the coldest, with some parts of southern England getting down to minus 9C, hovering around minus 7C in the early hours of Wednesday.' 

It comes as Mayor of London Sadiq Khan issued a high air pollution alert for the capital for today. 

Mr Khan said: 'We know how dangerous toxic air is for Londoners - that's why I'm doing everything in my power to tackle it. On Tuesday, alongside the extreme cold temperatures we've been experiencing, we are also expecting high levels of air pollution.

'Following the latest forecast from Imperial College London, I am issuing a 'high' air pollution alert. This shows once again why it's so vital that we expand the Ultra Low Emission Zone London-wide to reduce toxic air pollution in our city. 

'We all need to be careful over the next few days. I'm urging Londoners to look after each other by choosing to walk, cycle or take public transport where possible, avoiding unnecessary car journeys, stopping engine idling and not burning garden waste, all of which contributes to high levels of pollution. 

'This is particularly important in order to protect those who are more vulnerable to high pollution.'

Freezing fog in southern England appeared to thaw slightly with bright blue skies replacing the Arctic haze as temperatures crept above zero on Monday afternoon.

The mercury hit 1.3C in Wisley, Surrey, by 3pm — markedly warmer than the -8.4C recorded at Heathrow Airport during the coldest night since 2010.

But Britain is seeing something of a North-South divide when it comes to the climate, with Achnagart in the north-west Highlands recording a mild 12.1C. 

The variation is being caused by different types of air masses being swept in, with milder air being blown towards Scotland.

The freezing fog wreaked travel chaos on Monday morning, cancelling nearly 100 flights at London's busiest airport.

Conditions are set to even out by the end of the week, forecasters said.

Met Office spokesman Nicola Maxey said by Friday, the whole country will see 'slightly milder airspace' with temperatures rising to high single digits.

She said although 'not balmy' it will be 'typical for this time of year'.

But Ms Maxey said to expect more unsettled conditions, including wet and windy weather, into the start of February.

The disruption threatened to hit 12,000 passengers hoping to fly from Heathrow as the airport scrambled to reduce flights by up to 15 per cent due to Air Traffic Control restrictions.

Passengers reported being stuck on grounded plans for hours on Monday morning as the mercury plummeted to -8.4C overnight.

Around 80 British Airways flights were forced to cancel - as were dozens of other airlines - due to low visibility caused by fog descending overnight.

The start to the day felt bitterly cold in eastern England with temperatures plummeting to as low as -9.5C in Santon Downham, Norfolk, just before 8am.

It was also the coldest January night since -9.1C was recorded in 1987, according to forecasters. 

The Met Office issued a yellow fog warning overnight that lasted until 11am, covering southern and eastern England as workers battled with below freezing temperatures on Monday morning. 

Freezing fog is defined as such when visibility is less than 0.6 miles (1 km) and it is combined with an air temperature below 0C. 

Forecasters warned that low visibility could cause potentially hazardous road conditions, and that the fog could become so thick that visibility could drop to as low as 50 metres in some parts of England.

Flights in and out of Heathrow Airport were disrupted.

All airlines were affected but conditions lifted later in the morning.  

A spokesman for Heathrow said: 'Poor visibility is forecast this morning at the airport and across the South East.

'While there may be minor changes to today's schedule as a result of the weather, we want to reassure passengers that our colleagues are working in close collaboration with our airline and air traffic control partners to get them safely away on their journeys as quickly as possible.

'We encourage passengers to check with their airline for the latest information.'

British Airways said: 'Like other airlines, our schedule has been affected by the continued freezing fog weather conditions experienced across London.

'We've apologised to customers whose flights have been affected and are doing everything we can to get them on their way as quickly as possible'.

Despite icy road conditions, Highways England said there had been no accident reports due to weather conditions since midnight.  

But the weather did not deter hardy swimmers who donned bobble hats and gloves to take a dip in Hyde Park's Serpentine this morning.

By 8am the Met Office forecast temperatures were around -6C in the capital and would struggle to rise above zero until midday. 

Forecasters said it was unlikely to top 4C later in the day, while temperatures could reach 10C by mid-afternoon in Northern Ireland.

Tonight is expected to bring a mixture of conditions with mostly clear skies in the south but heavy cloud further north.

Sunday saw mixed weather conditions across the UK as milder air moved into northern and western parts, while southern and eastern areas remained cold.

The Royal Parks Police said it had received reports of children walking on ice in Bushy Park. 

A video also emerged of a dog being rescued by a fisherman after falling through the ice, prompting them to put out a safety warning. 

More than a million households will be paid to cut back their electricity tonight as part of an emergency scheme to prevent blackouts on one of the coldest days of the year.

The National Grid is tipped to reward those participating in the Demand Flexibility Service as they voluntarily reduce their usage between 5pm and 6pm, preventing the nation's supply from being overstretched.

It is the first time National Grid has implemented its DFS scheme since it was first announced last November, The Times reports, as the UK prepares for freezing temperatures this week.

In a further sign of the squeeze on supply, the company is preparing to use its back-up coal plants at the Drax power station in North Yorkshire and West Burton in Nottinghamshire today.

It will be the first time the two coal-fired power units are warmed up on standby, ready to generate in case they are required.

A spokesman from National Grid's electricity system operator said: 'Our forecasts show electricity supply margins are expected to be tighter than normal on Monday evening.

'We have instructed coal-fired power units to be available to increase electricity supplies should it be needed tomorrow evening.

'This does not mean electricity supplies are at risk and people should not be worried. These are precautionary measures to maintain the buffer of spare capacity we need.'