But he was condemned by the Conservatives, who said he should have given details of the case to MPs before they were revealed in newspapers.

It does seem amazing that the prime minister ... wasn't even told about the Mitrokhin case until a day before it broke in The Times

Ann Widdecombe

Mrs Norwood, an 87-year-old great-grandmother, passed British secrets to the Russians in a 40-year association with the KGB.

The information she gave included developments in the UK's atom bomb technology. The revelations of her spying caused a sensation when they were published last year.

'Effective and adequate'

Earlier this month, a powerful group of MPs accused MI5 of serious misjudgements in not even interviewing the former Soviet agent with a view to prosecution.

The Intelligence and Security Committee concluded that MI5's decision not to refer the case to government law officers was a "serious failure".

Mrs Norwood passed on secrets of atom bomb development

The committee found that Mrs Norwood escaped prosecution because a junior MI5 officer decided the case was not worth pursuing.

Mrs Norwood's role was revealed by KGB archivist Vasili Mitrokhin when he defected to the West in 1992, but no action was taken.

Current MI5 director Stephen Lander has admitted that the secret service had made a mistake in not launching a prosecution.

During a Commons debate on the country's intelligence and security services, Mr Straw said he was confident the services were effective and accountability and oversight were adequate.

'Lessons learned'

Mr Straw said: "As the inquiry report made clear, and as the government acknowledged in its response, mistakes were made in the handling of this material and not least in the decisions that were made and not made as to whether or not individuals should be prosecuted.

"The agencies as well as the central departments concerned - the Foreign Office, the Home Office and the Cabinet Office - have learnt a great deal from the report and this experience.

"I believe that the lessons have been identified and accepted."

He also disclosed that when Mrs Norwood's spying became public knowledge, he had avoided revealing details in his initial statement because he was "groggy" after an operation.

Media revelations

Shadow home secretary Ann Widdecombe condemned Mr Straw for not revealing details of the case to MPs before they were revealed in the press.

"One of the most unfortunate aspects was that we had to learn about the whole affair through leaks to the media," she said.

The spy lived in an ordinary, suburban house

"It does seem amazing that the prime minister ... wasn't even told about the Mitrokhin case until a day before it broke in The Times," she said.

She said even news of the decision not to prosecute Mrs Norwood was made to the media and not to the Commons, so the handling of the case by the government and MI5 was deficient, she said.

"I personally find it regrettable that confessed traitors such as Melita Norwood will never be prosecuted for betraying their country," said Miss Widdecombe.

"There were undoubtedly serious failings in the system and we can all hope those won't recur."

And she accused government departments of a "complete lack of co-ordination".

She paid tribute to the "good work" of the committee.

Vasili Mitrokhin's records led to the spy revelations

The Intelligence and Security Committee, chaired by former defence secretary Tom King, is parliament's watchdog for M15, M16 and the secret communications centre, GCHQ.

Last year, Mr Straw announced that existing arrangements for oversight of the security service would be tightened and that it would be required to provide an annual report on all its current spy cases.

But the home secretary praised Mr Lander and told MPs his appointment had been extended until October 2002.