Nicola Bulley's dog likely went into "fight or flight mode" when she went missing, according to a canine behaviour expert.

There has been no sign of the 45-year-old mum of two since Friday, January 27, when she was last seen walking her dog on a path along the River Wyre in St Michael's on Wyre, Lancashire.

On Friday, police revealed that the main theory they were working on is that Nicola may have fallen into the river.

Superintendent Sally Riley told a press briefing it was "possible" Nicola toppled into the river while trying to retrieve a tennis ball for her springer spaniel, Willow.

Shortly after her disappearance, Willow was found nearby by another dog walker and appeared to be "bone dry".

It sparked speculation that it could mean Nicola did not fall into the water, but dog behaviour expert Ross McCarthy claims it is not as simple as that, as he said "dogs don't jump in to save owners".

McCarthy told the Express: “It all depends on the dog’s association with water, and their relationship with the owner and so on, as to whether they would jump in.

“Some would through play or investigation and others would just mooch about on the side and some would panic at the strange situation."

He said that while "dogs go into fight or flight mode" like humans, they might not "perceive the same dangers as us".

“Often chemical changes in the body of the owner can alert the dog to an unusual situation but naturally if the owner is not present - that won't be the case," he said.

“Presumably this dog has been walked along the canal and water often and been discouraged from going in or naturally just avoided going in.”

McCarthy added: “It would be a strange occurrence from the dog’s perspective - but outside of Disney and in exceptional cases - dogs don't jump in to save owners.

“It's no different to when you jump in a swimming pool - the majority of dogs - unless conditioned otherwise will just circle the pool.”

Although police currently believe the hypothesis that Nicola slipped into the river to be the most likely scenario, officers have admitted there is "no evidence" to support the theory.

But they have also said that third-party involvement was "highly unlikely".

Nicola's sister, Louise Cunningham, took to Facebook after the police announced their theory to urge people to "keep an open mind" as she said that "there is no evidence whatsoever that she has gone into the river".

Nicola's close friend Emma White also disputed the theory, claiming that it was based on "limited information".

She told SkyNews that there was "definitely no ball" on the walk as Nicola had stopped taking them "way into last year" because of it disturbing her walks with Willow.

The search for Nicola is now into its 10th day.

Specialists continued to scour the river and surrounding area yesterday and police have now confirmed that a potential "key witness" to the disappearance has come forward.

It follows the Lancashire Constabulary's appeal last night for help tracing a woman seen on CCTV pushing a pram close to the River Wyre on the morning Nicola vanished.

Officers have said that she is "very much being treated as a witness" since coming forward.