Alison Davies and her son Ryan were last seen on Tuesday

Ryan, who was autistic and had a mental age of seven, went missing from Marple near Stockport with his 40-year-old mother Alison on 11 April.

Police said Ms Davies, who is still missing, may have travelled to Hull with the intention of harming herself.

A spokeswoman said officers were examining CCTV footage which showed "an incident" on the Humber Bridge.

Police fear Ms Davies, who suffers from depression, may have fallen from the Humber Bridge with her son.

If she is still alive I would urge Alison to contact police as a matter of urgency Det Supt Colin Andrews

The Coastguard recovered Ryan's body from the river at Swinefleet Haven, near Goole, East Yorkshire after being alerted by the skipper of a pleasure boat on Sunday.

Police helicopter and lifeboat searches of the river were carried out on Monday afternoon but did not find anything.

CCTV has been recovered from the bridge and officers are examining images of an incident on the walkway on 12 April, one day after the pair were last seen at Ms Davies' mother's house more than 70 miles away.

Ryan's body was found more than 70 miles from his home

But they said they were keeping an open mind as to whether the incident was connected to Ryan's death.

Detective Superintendent Colin Andrews, of Humberside Police, said they were desperate to find Ms Davies who has friends in the city and used to live there herself.

"We do not believe Alison had visited Hull for some time but we need to speak to anyone in the area who may have had contact with her recently," he said.

"We must also establish how Alison and Ryan got to Hull and so I would appeal to bus and taxi drivers in the city to think back to last week - do you recall taking a woman and her son to the Humber Bridge?

"Given what has happened to Ryan we are concerned for Alison's safety and as time passes that concern grows and the likelihood of finding her alive and well reduces significantly.

"If she is still alive I would urge Alison to contact police as a matter of urgency."

Ms Davies is described as white, 5ft 4in tall and of slim build. She has shoulder-length brown hair.