MI5 head: 'We cannot stop every terrorist plot and 7/7 could not have been prevented'

He referred to a later plot in which a British-based terror cell plotted to blow up airliners using home-made liquid bombs.

"A year later we detected and, with partner agencies, prevented al Qaida's most ambitious plot - to bring down multiple airliners on US cities using liquid bombs on flights from London," Mr Parker said.

"Thousands would have died. I'm not sure we would have detected it without the uplift that followed 7/7."

MI5 was criticised by a coroner at the end of an inquest into the deaths of the 52 people who died when suicide bombers attacked London's transport system.

She raised concerns about how the service investigates and prioritises suspects but rejected claims that security agency failings caused or contributed to their deaths.

In a statement to mark the 10th anniversary of the outrage, Mr Parker, who had taken over as counter terrorism director a few months earlier, said: "We had always known - and said publicly - we simply can't find and stop every terrorist plot.

"We could not have prevented 7/7. While it remains true that we thwart most attempts, the rare occasions when terror attacks occur stand as stark moments in contemporary history."

The Director General of the Security Service said the "continuing fact" that some British citizens are prepared to target their own country presents a "serious societal and security challenge" 10 years after the attack.

He said: "These and other appalling acts are attempted by individuals who have grown up here but decided for whatever twisted reasons to identify their own country as the enemy.

"They are a tiny fraction of the population. But the continuing fact that some people, born in the UK, with all the opportunities and freedoms that modern Britain offers, can nonetheless make those sorts of warped choices presents a serious societal and security challenge.

"The terrible events in London on 7 July 2005 are enduring reminders of the reality of what MI5 is striving every day to prevent."

Mr Parker said the "shocking and brutal" murder of holidaymakers in the Tunisian beach massacre has "yet again brought the reality of terrorism to the fore", adding that on Tuesday Britain remembers "the victims of another dreadful day".

Additional reporting by PA.