Grandfather John White, 48, of Allaway Avenue, Paulsgrove, was going for a morning walk with his wife and dog when a woman ran out of the bushes claiming a swan had attacked her child.

She pointed to the nearby lake where Mr White saw a baby, clothed and with shoes on, floating face down in the water.

Mr White, who suffers from angina, immediately sprang into action.

He took off his jacket and jumped into the filthy, waist-deep water while his wife and dog waited nearby just out of sight.

But when he grabbed the child's leg he realised it was actually a doll.

When Mr White turned around he saw the woman and her male accomplice stealing from his coat.

As he attempted to get back to the path he was squirted in the face with a substance from a plastic bottle, which burned his nose and eyes.

The thieves then fled east through Foxes Forest, in Hilsea, Portsmouth, with Mr White's tobacco and more than £100 in cash.

Both Mr White and police blasted the thieves as 'sick'.

Mr White said: 'I felt stupid that I fell for a trick.

'I thought I was trying to help someone. It even made me feel like next time someone asks for help, I'd just walk past.

'They're playing dangerous, dangerous games. I think they're sick. I've been thinking about it, and either they're desperate for money to feed some addiction, or they're just thrill-seekers out to see what they can get.

'I don't think it would've mattered to them if it was me or a pensioner.

'They probably would've struck the first person they saw.

'If anyone did see them, let police know.

'They need to be caught before they go one step further.'

The bizarre theft happened at around 10.30am on Wednesday, and the doll has been taken by police as evidence.

Detective Constable Chris Dinenage said: 'This is a rare and unique incident that has happened in a well-used recreational area of Portsmouth.

'The method used by the offenders is quite sickening and the victim acted in good faith to assist a seemingly helpless member of public.'

The man is described as slim, aged 20 to 25, around 5ft 7ins and wearing a beanie hat and a dark zip- up top.

The woman was a few years older, around 5ft 2ins, wearing a sky-blue hoodie, with short brown hair cut in a bob.

Anyone with any information should contact Det Con Dinenage at Fratton police station on 101, or Crimestoppers on 0800 555 111.