The head of MI5 has resigned weeks before full details of the role of her agents in a surveillance operation involving two of the July 7 bombers are due to be revealed.

Dame Eliza Manningham-Buller, whose organisation has been at the forefront of the war on terror, is leaving after more than four years as director general.

Dame Eliza, 58, said the date of her departure after 33 years with the security service had been agreed with the former Home Secretary Charles Clarke, who was sacked in May.

She has maintained an unprecedentedly high profile in the fight against terrorism, revealing last month that the security services knew of 30 plots by Islamic extremists. But it is for the failure to prevent last year's attacks in London that, some believe, her tenure as MI5 chief will be remembered. Mohammed Siddique Khan and Shehzad Tanweer, the leaders of the July 7 suicide bombers, were picked up by MI5 surveillance on five occasions but were not investigated further.

The two British-born bombers did not merely pass through the 'periphery' of an intelligence operation monitoring other suspects but were photographed and recorded on several occasions.

More details of the operation are likely to emerge in the New Year.

Intelligence sources say the men were first seen in early 2004, nearly 18 months before the suicide attacks in London, which left 52 people dead on three Underground lines and a bus.

On one occasion, Khan was monitored driving his car with suspects in it and on another was recorded talking to them about training for jihad.

They also talked about carrying out financial frauds, which helped persuade MI5 that they were not interested in attacks in the UK.

Last night security sources rejected suggestions that Dame Eliza jumped before she was pushed.

They stressed that she agreed her departure date - April 2007 - with Mr Clarke in 2005, before the July bombings.

They claim there will be 'no surprises' that might have called her position into question when further details of the surveillance operations enter the public domain.

'Everything there is to know about how MI5 handled the 7/7 bombings, and what happened before, has been presented to the Commons Intelligence and Security Committee,' a source said.

'There are no surprises in store that will alter the view of how the Security Service worked. Her departure is a routine event, longarranged. The Home Secretary has full confidence in her.'

Dame Eliza, who is paid £150,000 a year, took over counterterrorism operations a year after the

attacks of September 11, 2001, and has overseen a transformation in MI5 as its budget and staff have increased to focus on the threat of Islamic extremists.

She has made the agency far more open, recruiting agents through newspaper advertisements and by setting-up a website. Terror risk assessments have been published for the first time.

Dame Eliza said recently that the security services had identified 1,600 people plotting actively, or facilitating, terrorist acts in Britain and abroad.

The daughter of a former Tory Lord Chancellor, she is described as a 'feisty lady, full of character and intellectual drive'.

She was chosen to run a unit set up to tackle Irish terrorism after MI5 was granted lead responsibility in the area ten years ago

Tony Blair led tributes to Dame Eliza, highlighting her ' outstanding leadership' following July 7 and saying the country owed her a debt of honour.

In a statement, the Prime Minister said: 'Dame Eliza Manningham-Buller has dedicated herself to the protection of this country, our people, and our way of life.

'She has led the Security Service through a time of significant change and growth, as it responded to the challenge of international terrorism.'

Home Secretary John Reid, who will announce her replacement in the New Year, said: 'Her contribution to the security of our nation has been invaluable.'

In a statement, Dame Eliza, the second woman to head MI5 after Stella Rimington, said: 'By April 2007, I shall have been an officer of the Security Service for 33 years, the last ten as either deputy director general or director general.

'I decided in early 2005 that it would be time by then to stand down.

'I have been privileged to lead the service when it is facing the two challenges of a very serious threat and the consequent need to grow and change at a dramatic rate to tackle that threat.'

'I'm confident that the service will continue to serve the UK to the best of its ability.'