An anti-war protest that saw two Greenpeace members scale Big Ben was a huge breach of security, a defence analyst has said.
The two protesters unfurled a banner
Greenpeace said its members scaled two security fences around the Houses of Parliament in order to climb Big Ben at 0615 GMT on Saturday.
A spokesman for the Metropolitan Police said officers reacted
immediately after an alarm went off when the pair climbed the fence next to the tower.
"We are confident that at no stage were the protesters in a position that they could have gained access to the inside of any building within the Palace of Westminster," he said.
But Humphrey Cram Ewing, a defence and security analyst who regularly attends the Palace of Westminster, told BBC News Online: "Big Ben is a high profile target which you would expect terrorists to go for.
"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."
The BBC's Clarence Mitchell said an intense police investigation was under way.
He said a police source commented that if it had become clear the protesters had any terrorist intent the response would have been "vastly different".
Mr Cram Ewing suggested the group may have had inside information.
He said if the two men managed to get across the two security fences in front of St Stephen's Tower - which houses Big Ben - without detection this was "very surprising".
"As far as I am aware the inside of one of these fences is electro-sensitive and both fences should have been watched by closed circuit television," he said.
Mr Cram Ewing, an associate fellow of the Royal United Services Institute, said the timing of the protest was significant.
"Security may have been looser when the Houses of Parliament are shut, but that is exactly when additional vigilance and tightness is required," he said.
Glen Smyth, chairman of the Metropolitan Police Federation, condemned Greenpeace's actions and said police behaved in a very professional manner.
"Greenpeace should be ashamed of their antics," he said. "I refute absolutely any allegation that the security was breached in any meaningful way."
Public access to the Houses of Parliament is something which over the years has been fiercely defended by MPs.
There are also some 20 categories of passes which allow access to the Palace of Westminster.
The two men, aged 23 and 28, and both from Lewes, East Sussex, arrested on suspicion of causing criminal damage but were later released on bail to return to a London police station in April.