ROSIE Boycott, the editor of the Daily Express, has left the paper ''by mutual agreement'' ending a brief and acrimonious relationship with its new owner.

Ms Boycott, arguably the most successful female newspaper editor in history, and her deputy Chris Blackhurst, have been in negotiations for the past six weeks about their future after Richard Desmond, the soft porn and celebrity magazine publisher, bought the Express, Sunday Express and Daily Star in a #125m deal two months ago.

A joint statement from the company and Ms Boycott said the pair, who have held their positions for the past two years, would leave with ''immediate effect''.

Chris Williams, the paper's former associate editor, will become the new editor.

It has been widely publicised that Ms Boycott, a leading feminist and the former editor of the Independent and Independent on Sunday, became concerned about the direction of the paper.

She is said to have been unable to see eye-to-eye with Mr Desmond, who is said to revel in the title ''people's pornographer''.

Doubts about her future arose as soon as Mr Desmond, whose company Northern and Shell also publishes OK! and several pornographic magazines, bought the titles and changed the Express's front-cover story without her consent to promote OK!

Their relationship deteriorated when she refused to supply a ''redundancy list'' of journalists. The pair are understood not to have spoken since December 15.

Yesterday, Ms Boycott said: ''I would like to wish the newspaper, its staff and the new proprietor well in their endeavours to build on the great name and tradition of the Daily Express.''

She said she was ''fantastically proud'' of the work she had done with the paper but said she wanted a new start.

''I think Richard Desmond wants a new start. I wish him all the best of luck with the paper. We are parting extremely amicably. We had an amicable divorce and, as with all good divorces, the details will remain confidential. I adore the paper so I am sad to go.''

Ms Boycott became the first woman to edit a national broadsheet when she took over the Independent on Sunday in 1996.

Before that she spent four years as editor of men's magazine Esquire and almost doubled its circulation. She won an editor of the year award in 1993 and 1994.

Chris Williams, the new editor, began his journalistic career with the Press Association before joining the Daily Express in 1977 where he worked for 10 years, rising to the rank of features editor.

In 1987 he joined the Daily Mail, also as features editor, before returning to the Express in 1995 as executive editor.

Paul Ashford, the editorial director of Northern and Shell, said Mr Williams was ''an experienced and talented journalist whose appointment ensures the editorial continuity of the newspaper''.

Mr Williams said: ''It has always been my ambition to edit the Daily Express and it is a particular privilege to do so at this exciting time with a new proprietor and new challenges.''

Kerry Gill, the Scottish editor of the Express, said he welcomed the change. He said he expected it to have no effect on the 50 jobs in Scotland.

Roy Greenslade, a media commentator, said Ms Boycott's departure was ''hardly unexpected'' although he said he was ''baffled'' as to Mr Desmond's long-term plans for the group of titles.

Ms Boycott and her deputy are the latest in an exodus of senior staff from the Daily Express. Peter Hitchens, the paper's most senior columnist, resigned in December after 24 years, along with Anthony Bevins, the political editor, and several other top writers.

Mr Desmond said he wants Express sales, which have sunk to a little more than 1m copies, to eventually overtake the Daily Mail, which sells about 2.5m daily. He succeeded in turning OK!, a loss-making title, into the market leader by paying out huge sums for celebrity exclusives, such as the #1m reportedly spent on David and Victoria Beckham's wedding.

But he still had to borrow #97m to buy Express newspapers, and has embarked on a savage cost-cutting programme since taking over the Express titles.

He has also put his portfolio of soft porn magazines up for sale.