The bereaved relatives and those who escaped the bombings delivered a letter to Home Secretary Jacqui Smith on Wednesday calling for the government to re-think its refusal to hold an inquiry.

They said they would seek a judicial review should their request be denied, arguing the decision would breach the European Convention on Human Rights.

Opposition politicians have echoed such calls but Smith's predecessors have previously dismissed them, saying an inquiry would distract the already-stretched security services at a time when the country is at serious risk of attack from militants.

Britain is currently at its second highest threat level, "Severe", meaning an attack is highly likely.

In June, a jeep laden with petrol canisters was rammed into Glasgow Airport, just days after two cars packed with gas canisters, petrol and nails were found in London.

"As we have consistently maintained, experience has shown that a fuller public inquiry can take years and divert huge resources," a Home Office spokeswoman said, adding that a parliamentary committee was carrying out further probes.

Fuelling the group's demands for an inquiry has been the revelation since the deadly 7/7 bombings that two of the men had been monitored by security services a year earlier, raising the suggestion the bombings could have been averted.

Shehzad Tanweer and Mohammad Sidique Khan were photographed, recorded and followed by intelligence operatives several times in early 2004 in the company of plotters who have since been jailed for planning attacks using fertiliser-based bombs.