Tony Blair's director of communications, Alastair Campbell, 46, has resigned -- a week after giving evidence to the Lord Hutton Inquiry, which is probing the death of government scientist Dr David Kelly.

"It has been an enormous privilege to work so closely in opposition and in government for someone I believe history will judge as a great transforming prime minister," Campbell said in a statement posted on 10, Downing Street's Web site.

"We agreed on April 7 of this year, however, that I would definitely leave this summer and I have now given the prime minister formal notice of my decision to leave."

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He added that he would be available to assist the inquiry.

"I did not think it appropriate to announce this on a day when Lord Hutton was sitting... I will be available in the next few weeks to assist the handover to my successor, who will be announced shortly."

Blair said in a statement: "I knew for a long time he wanted to move on and I wish him every success. He was, is and will remain a good friend."

No date has been set for his departure, Downing Street said.

Campbell is involved in a bitter row with the BBC over its story that the government 'sexed up' a dossier on Iraq's weapons of mass destruction.

Dr Kelly, exposed as the main source of the BBC report, was found dead on July 18.

On August 19, Campbell told the Hutton Inquiry that he had not tinkered with the dossier.

"I had no input, output, influence upon it whatever at any stage in the process," he said.