The superheroes of the Fathers4Justice campaign group have hung up their capes after the discovery of a plot to kidnap Tony Blair's five-year-old son.

The plot by militant members or former members of the group never got off the drawing board and appears to have been firmly quashed by Special Branch officers who warned the alleged plotters that they could be shot if they entered Downing Street.

But the founder of the group said tonight that he was closing it down in disgust at the news and a former member who scaled Buckingham Palace in 2004 said the July 7 bombings in London meant high-profile stunts in the capital were now simply too risky.

Matt O'Connor, the marketing executive who founded the group in December 2002, expressed his outrage after The Sun reported a foiled plot to abduct Leo Blair, the Prime Minister's youngest child.

Mr O'Connor said he had no knowledge of the plot but believed that Scotland Yard anti-terrorist officers had visited former members of the organisation over the Christmas break and told them that they could be shot if they attempted any action against the Blairs.

Mr O'Connor told Channel 4 News tonight that the group can no longer continue in light of the negative publicity generated by the allegations. He said: “I regret to say that three years after starting the organisation we’re going to cease and bring it to a close.”

Jason Hatch, who dressed up as Batman and climbed onto a Buckingham Palace balcony during a protest in September 2004, denied a claim by Mr O'Connor that 30 militant members of the group, including himself, had been thrown out last May. He said they had walked out after deciding that Mr O'Connor was not fit to lead the organisation.

"We've had enough of the high-profile stuff," he added. "After the terror attacks, the police will shoot you in London now."

It is not clear how far the extremists got with their plot to abduct Mr Blair's son. Police sources said last night that the kidnap plan was one of many being discussed by the splinter group, but emphasised that it was at an early planning stage.

No arrests have been made, although Stewart Tendler, Crime Correspondent of The Times, said that the plot had been taken seriously and exhaustively investigated. "Police sources said that officers had been deployed to examine the threat and decided that the kidnap plot would not have been feasible," he said.

Downing Street has refused to comment on the plot, although a security source told The Sun: "They were naturally very concerned, as any parent would be. But they have been assured the police are on top of the situation."

Mr Blair’s official spokesman said today: "We don’t talk about security matters, and all I will say is this story illustrates precisely why we are wise not to talk about security matters.

"There are real security concerns. We have to address them and the best way to address them is not by talking about them."

Police have since carried out a security appraisal of the protection given to the Blairs and their other children, Euan, 21, Nicholas, 20, and Kathryn, 17.

Fathers4Justice was set up by Mr O'Connor after he found it difficult to see his two sons after a bitter divorce. Its first stunt, in December 2002, involved 200 men dressed as Santa Claus raiding the Lord Chancellor's department.

The following year one of the group's members caused major traffic jams in Central London when he dressed up as Spiderman and scaled a crane on Tower Bridge, where he spent five days.

In 2004, the group staged its two highest-profile stunts, when two men invited into the Commons by an MP threw condoms full of purple flour at the Prime Minister, and Mr Hatch in his Batman costume scaled the walls of Buckingham Palace.

Mr O'Connor has described his group as unique, because it uses humour to draw attention to the plight of men separated from their children by the family courts, which it says favours mothers.

"If we dressed normally, we would have as much impact as a bus queue," he once said. "And although we are just middle-aged men in unflattering outfits, we try to be iconic - like Mel Brooks's Men in Tights, or The Incredibles."

But its stunts in the past have turned off as many people as they have entertained, and the group's success in forcing a serious debate on fathers' rights has been limited.

Mr O'Connor said today: "I'm very angry and upset that this organisation which has, I believe, contributed to the debate and raised awareness of the plight of children denied access to their fathers by the family courts has been undermined by the very people it is meant to help.

"It's profoundly damaging for us as a campaign. It's damaging for the cause we represent."

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