MI5 had no "realistic prospect" of uncovering the 7/7 plot even if it had followed up clues about the bombers, the inquest into the attacks heard today.
A senior officer with the security agency said he did not believe more steps should "reasonably" have been taken to investigate ringleader Mohammed Sidique Khan and his deputy, Shehzad Tanweer.
Counter-terrorism officers watched the pair meeting a group of extremists planning a fertiliser bomb attack in early 2004 but did not identify them at the time, the inquest has heard.
The top MI5 officer, identified only as "Witness G", insisted today that there was not enough intelligence to make Khan and Tanweer priorities for surveillance before the July 7 2005 London bombings.
James Eadie QC, counsel for the Home Secretary and MI5, asked him: "Knowing what you know now, do you consider that more steps should reasonably have been taken to investigate in particular Mohammed Sidique Khan and Tanweer?"
Witness G answered: "I do not."
Mr Eadie went on: "Even if further steps had been taken, do you consider there to have been any realistic prospect of uncovering the 7/7 plot?"
Witness G again replied: "I do not."
The inquest is looking at whether MI5, also known as the Security Service, could have drawn together different strands of intelligence about Khan's links with extremists and established that he was planning mass murder.
Many 7/7 survivors and relatives of those killed in the attacks argue that MI5 had enough information about the bomber to make him a priority for an in-depth investigation which would have uncovered his plot.
The inquest heard this week that Britain's security agencies missed a series of opportunities to learn more about Khan, including:
:: A clear colour photograph of Khan and Tanweer taken by an MI5 surveillance team was never shown to al Qaida supergrass Mohammed Junaid Babar, who met Khan at a terrorist training camp in Pakistan in 2003 but did not know his real name;
:: MI5 admitted it could have identified Khan as an extremist who had undergone military training in Pakistan but did not follow up intelligence it received in early 2005 for undisclosed reasons relating to national security;
:: A Security Service officer suggested in May 2005 that a group of northern men - including Khan and Tanweer - seen meeting the fertiliser bomb plotters might be among the unidentified British jihadists Babar met in Pakistan.