Despite a leaked secret report dated last October, three months after the attacks, which indicated that little had been learnt of the background to the plot, more recent discoveries have produced firmer links between the four British bombers and al-Qaeda.

The biggest question for MI5 and police has been whether the London bombings were conceived and planned entirely in Britain or whether there was outside influence, some form of mastermind supplying expertise, money and planning.

Security sources said for the first time yesterday that the intelligence services were closer to answering the key questions, providing evidence of a foreign connection, linking the London bombers to terrorists in Pakistan.

The results of the six-month inquiry, which has included visits to Pakistan by MI5 officers and police counter-terrorist specialists, will be laid out in a detailed report in April by the parliamentary Intelligence and Security Committee, headed by Paul Murphy, the former Northern Ireland Secretary.

Security sources said yesterday that “enormous progress” had been made since October, when, according to the document published in The Sunday Times, there were still huge gaps in intelligence.

Sources said that the report was based on an analysis of the facts and at that time there were no real clues about a foreign connection other than the known visits to Pakistan by two of the suicide bombers.

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