Charles Clarke has been appointed as education secretary following the resignation of Estelle Morris.

Clarke, the party chairman, is a tough political operator who has served time as a junior minister in the department of education.

Announcing the decision, Number 10 made clear Clarke had been picked because he was an experienced minister and "a big figure in every sense".

"The prime minister and he go back a long way. The prime minister is fully confident in his abilities to manage big issues," said the spokesman.

With praise came a warning that the new job will be no easy ride.

"He now has to build on the fundamental transformation brought about by both David Blunkett and Estelle Morris," said Number 10.

Clarke's post as party chairman has gone to John Reid, who is replaced as Northern Ireland secretary by Paul Murphy, formerly the Wales secretary.

Reid was given the job because of his proven role as a media heavyweight and his experience in several government departments.

"John Reid is someone who is vastly experienced in talking about government policy across a range of issue."

The current problems in Northern Ireland will not be affected by the appointment, said Downing Street, pointing out that every change was always seen as being during a crisis.

Murphy is, according Number 10, well-known in Ulster and has a reputation for being "fair-minded".

"Paul Murphy is not only experienced in Northern Ireland but is also genuinely respected by all the parties in Northern Ireland," said the spokesman.

He, in turn, is replaced at the Wales Office by the former Europe minister, Peter Hain.

Morris resigned on Wednesday night after criticism of her handling of the A Level re-grading fiasco and a series of other rows.

The education secretary quit saying she did not feel up to the job.

She had faced Conservative allegations of misleading parliament over a pledge to resign if key numeracy and literacy targets were not met.

An exchange of letters between Morris and the prime minister indicated that it was her personal decision to stand down rather than face continual criticism from opponents and the media.

The letters revealed she had offered her resignation on Tuesday, but the prime minister had asked her to think again overnight.

On Wednesday evening Westminster was stunned by the news that she had written to the prime minister to confirm her wish to resign.

In a remarkably personal resignation letter, Morris indicated she felt the post of education secretary had been beyond her abilities.

"As I explained to you when I came to see you yesterday morning, I am proud of the role I have played in government, both as schools minister and secretary of state.

"In many ways I feel I have achieved more in the first job than I have in the second. I have learned what I am good at and also what I am less good at.

"I am good at dealing with the issues and in communicating to the teaching profession. I am less good at strategic management of a huge department and I am not good at dealing with the modern media."

She told the prime minister that her limitations meant she was not being "as effective as I should be or as effective as you need me to be".

"You were kind enough to say that you wanted me to think about it further overnight and be absolutely sure that this is what I want to do. I have done so and it is."

Downing Street denied putting any pressure on Morris to leave the government.

Tony Blair's letter to Morris said she had been doing "an excellent job".

"Politics can be a tough and lonely job and the pressures can be intense," he said.

"I'm sorry that things have not worked out for you as both of us hoped and wished when I appointed you.

"I have no doubt at all that you will come back into government. In the meantime I want you to know that I continue to hold you in the highest regard.

"You have been a great asset to this government and can be very proud of your work."