A senior cabinet minister has demanded a report into how activists breached Westminster security to climb Big Ben during anti-war protests on Saturday.
The activists had to scale a wall and security fence before their climb
Two Greenpeace protesters scaled Big Ben on Saturday morning, unfurling a banner saying "Time for Truth".
Leader of the Commons Peter Hain told the BBC's Breakfast with Frost the stunt raised serious security concerns.
He said: "It is a huge embarrassment, both to the House authorities and the policing arrangements.
"Twenty or 30 years ago an audacious protest like that by Greenpeace would
have been seen for exactly that. But what if these had been suicide bombers?"
He added: "I am very disturbed about it and I want an urgent report as leader of the
Commons about how it happened."
The activists, brothers Harry and Simon Westaway, 23 and 28, from Lewes, East Sussex, cleared a wall and security fence before climbing the 315ft clock tower.
They then staged a six-hour protest before being forced to descend because of strong winds.
The pair were arrested on suspicion of causing criminal damage but later released on bail to return to a London police station in April.
The two protesters unfurled a banner
Both men described overcoming Parliament's defences as "easy".
Peter Hain said he had been concerned about Westminster security since taking up his position and had met the director general of MI5 on a number of occasions.
"Indeed only last week we had a detailed planning meeting to have a proper security review, to make sure that we are absolutely certain that the House can be properly protected."
Tory defence spokesman Keith Simpson said it was a "wake-up call" for security and a "dangerous embarrassment" for Parliament and the police.
Liberal Democrat home affairs spokesman Mark Oaten said it raised "obvious questions on security".
Metropolitan Police Commander Brian Paddick has announced there will be an "automatic review" of security.
But he insisted there was never any danger of the men getting inside the Palace of Westminster.
"Yes, they could have left something there and run away but at the end of the day they could not have caused any breach of security in terms of threat to life, in terms of blowing up anything of importance just inside that perimeter fence."
He said the alarms were triggered as they should have been when the men first climbed the fence.
But Humphrey Cram Ewing, a defence and security analyst who regularly attends the Palace of Westminster, told BBC News Online: "It really means security is not good enough - even with considerable barriers, controls and police patrols.
"If two seemingly innocent people can get up there to hang a banner, then terrorists could plant a mobile phone and set this to blow up Big Ben."
Extra defences already planned at the Commons include a bullet and blast-proof glass shield in front of the Strangers' Gallery.
Concrete blocks were placed around the building last May and a new £5m-plus security and reception area is being considered.