UK faces biggest threat to its security since 9/11 attacks, says Home Secretary Theresa May

London: Britain faces a bigger threat to its security from terrorism than at any time before or since the Sept. 11 2001 attacks on the United States, Home Secretary Theresa May said on Monday.

The government will introduce new counter-terrorism legislation on Wednesday to tackle a range of threats, May said, particularly from Britons who have returned from fighting in Syria and Iraq alongside Islamist militants.

"When the security and intelligence agencies tell us that the threat we face is now more dangerous than at any time before or since 9/11 we should take notice," she told an audience in London.

May said that since 52 people were killed when four young Britons carried out suicide bombings in London in 2005, around 40 terrorist plots had been disrupted.

These included attempts to conduct Mumbai-style gun attacks on British streets, a plot to blow up the London Stock Exchange, plans to bring down airliners and conspiracies to murder a British ambassador and military servicemen.

"Almost all of these attacks have been prevented," said May. "But as the IRA once boasted, the terrorists only have to be lucky once."