The Sun newspaper says an undercover reporter smuggled materials for a fake bomb into the Commons only a day after hunt protesters forced their way in.
The Sun says references used by its reporter were not checked
The newspaper said its journalist had been working for three weeks in the catering department at Westminster.
Sun managing editor Graham Dudman thinks the stunt exposed "very serious" failings in security at the Commons.
Eight protesters who tried to barge into the Commons chamber at Westminster have now been released on police bail.
Commons leader Peter Hain is reportedly "absolutely furious" about the latest breach of Westminster security.
Mr Dudman said reporter Anthony France was able to get a job as a waiter using false references - which were not checked in any way but "couldn't have been more bogus".
The materials he smuggled in in the midst of heightened security on Thursday included batteries, wire, a timer and modelling clay.
Mr Dudman said it "couldn't have been anything other" than a bomb, the only difference being it was 20g of modelling clay instead of semtex.
Photographs of the reporter also show him serving tea to Deputy Prime Minister John Prescott while at Westminster.
Mr Dudman told the BBC's Today programme: "It just simply beggars belief that on the day that ... everybody was saying: 'I can assure you we are going to ratchet up security, it's never going to be tighter', that same day a man from the Sun walks in carrying what could have been a bomb.
"It's simply unbelievable."
The reporter's picture had already been on the paper's front page after he smuggled a fake bomb onto a plane at Birmingham International Airport.
Mr Hain said the stunt had confirmed all his worst fears about security in the Commons, exposing "the amateurish and old-fashioned culture which threatened the very cockpit of our democracy".
"It's not just government ministers at risk but the 14,000 other people who work in the Palace of Westminster who are at risk in this modern age of suicide
bombers," he said.
"I have immediately asked the security services to close the loophole exposed by the Sun."
Mr Hain has called for the appointment of a central director of security to oversee the protection of Parliament, to take over responsibility from the serjeant-at-arms and his staff.
Shadow leader of the House Oliver Heald described the latest breach as a "scandal" and said security checks needed to be tightened.
The Liberal Democrats also said Commons security needed an overhaul, with a greater role for Scotland Yard.
"This completes Britain's worst week of security blunders," said Lib Dem home affairs spokesman Mark Oaten.
And Metropolitan Police chief Sir John Stevens has urged a review of the rule banning police from entering the Commons chamber without being invited by the serjeant-at-arms.
But former speaker Lord Weatherall defended the serjeant-at-arms.
"I think it is utterly wrong to blame him for lapses of this kind," he told Today. "If one has, as I understand, an insider helping people get around security, it is a very serious matter and something's got to be done."
"Stunts of this kind can always be perpetrated, one never hears of people who fail to get in because they have been found out."
Security was stepped up following Wednesday's Commons invasion
BBC chief political correspondent Mark Mardell said it seemed the reporter managed to smuggle in the equipment "when security was at its highest".
He said the claim was embarrassing for the House of Commons security services.
He said: "The government feels we need one central figure who's in charge of all of this."
He said the fact the reporter's bag was not searched was not unusual as a member of staff - but proper checks should have been made before he was given the job.
Police have released on bail eight protesters who were arrested on suspicion of forgery, burglary with intent to commit criminal damage and violent disorder after bursting into the Commons.
All eight must return to a central London police station in October.