Robert Novak, the conservative political commentator who blew the cover of Valerie Plame, a CIA officer, five years ago, has been diagnosed with a brain tumour and is being treated in an intensive care unit in a Boston hospital.

It is not yet known whether his tumour is malignant, however his assistant said that tests were currently under way.

The 77 year old journalist was criticised after being the first to publicly reveal the name of CIA operative Valerie Plame in a 2003 column.

His article was published eight days after Ms Plame's husband, Joseph Wilson, said the Bush administration had twisted pre-war intelligence to exaggerate the Iraqi threat.

An investigation into who leaked Ms Plame's name later resulted in Lewis "Scooter" Libby, a former senior aide to Vice President Dick Cheney, being convicted of lying and obstructing the inquiry.

Mr Novak, who has written a column in the Chicago Sun-Times since 1966, was only this month fined $50 after he hit a homeless man while driving his black Chevrolet Corvette in downtown Washington.

Mr Novak did not stop but claimed later that he had not realised that he had hit the man. A witness described the man as being splayed across the columnist's windscreen.

Mr Novak, who is also a commentator for Fox News, part of the News Corporation group, and parent of The Times newspaper, had travelled at the weekend with his wife to Cape Cod, Massachussetts to visit his daughter.

After feeling unwell on Sunday, he was taken to Brigham and Women's Hospital where he was diagnosed with a brain tumour.

In a statement issued yesterday by his publisher, Eagle Publishing, Mr Novak said: "I will be suspending my journalistic work for an indefinite but, God willing, not too lengthy period."

He added: "Doctors will soon begin appropriate treatment."

Kathleen Connolly, Mr Novak's assistant, said that he was currently being treated within the Boston hospital's intensive care unit but was alert and talking.

It is not yet known whether Mr Novak's condition can be connected with his car accident.

According to Dr Lynne Taylor, a neuro-oncologist at Virginia Mason Medical Centre in Seattle, doctors are advised to ascertain whether individuals who claim that they cannot remember, or did not realise, that they had hit something during a car accident had a brain tumour.

She said: "People get spatial and visual neglect of a certain part of their bodies and they don't realize they've done what they've done."

John Boehner, the House Republican leader, yesterday said of Mr Novak's diagnosis: "This is stunning news for anyone who follows American politics, and my thoughts and prayers go out to Bob and his family. I know Bob will confront this challenge with the same courage with which he has taken on the political establishment in Washington for decades. And with God's help, he will prevail in this fight."

Mr Novak's column still appeared in yesterday's Chicago Sun-Times detailing the battle between John McCain and Barack Obama: "In the contest for president, Barack Obama is a magnetic candidate supported by a disciplined, well-organized campaign. John McCain seems wooden, with a campaign that appears to be in shambles. Yet Obama's lead in the polls over McCain is fragile because he so far has not won the support of a majority of American voters."

One website, Humanevents.com, which is owned by his publisher and describes itself as 'Leading the Conservative Movement since 1944', many of Mr Novak's supporters left messages of goodwill for the columnist. While the vast majority of the comments were supportive, a handful were not.

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