The annual report of the Intelligence and Security Committee confirmed in October that the security services routinely investigate the background and behaviour of potential members of the government.

The report said as many as nine leading politicians fighting the last two general elections were branded security risks, with reports on them passed to the leaders of the two main political parties.

MI5 routinely vets potential cabinet ministers

On Monday Mr Blair said in a Commons written reply that it had "long been" the practice for the state security services to draw to the attention of a newly elected prime minister "any significant national security information" relating to members of his party.

"This arrangement has been endorsed by successive prime ministers and I am satisfied that it remains both necessary and proper," he said in the reply.

Mr Blair's unequivocal backing for the practice - long suspected by many MPs but only officially confirmed in the recent report - came as the Commons was debating the intelligence services, including MI5 and MI6.

During the debate, MPs raised the issue of scrutiny of the agencies and some expressed disquiet at MI5 keeping files on politicians and categorising them as security risks.

In his written reply Mr Blair said the report of the Intelligence and Security Committee did not "criticise or question the probity of this process", he said, but emphasised the heavy responsibility on MI5's Director General, Stephen Lander, to ensure the accuracy and relevance of the files.

"In its response to the report, the government made clear it was satisfied that the director general took this responsibility seriously and that the arrangements continued to be proper and appropriate."