The editors of the Daily Mirror and Sunday Mirror have lost their jobs as the two newspapers merge into one title.
Publisher Trinity Mirror announced that Richard Wallace and Tina Weaver had been made redundant and would leave the company "with immediate effect".
It added The People editor Lloyd Embley had been appointed to oversee both newspapers in the interim.
Mr Wallace told the BBC he "had been sacked" and "did not resign".
The publisher said two new editors will also be hired to run the weekday and weekend editions.
It added both roles would report to Mr Embley and the new appointments, together with a new editor for The People, would be announced "imminently".
A source at the Mirror told the BBC that Mr Wallace had been at work on Wednesday morning before he was sacked at around 10:00BST, adding the former editor "clearly had no idea" it would be his last day in the role.
Trinity Mirror said the decision to move to a seven-day publishing model was "a further step towards creating one of the most technologically advanced and operationally efficient newsrooms in Europe".
It added it would also launch new e-editions of its papers for tablet computers.
The change comes after tabloid rival The Sun launched a Sunday edition following the demise of the News of the World.
"I'm delighted to appoint Lloyd to the position of Editor of the Daily and Sunday Mirror," Mark Hollinshead, managing director nationals, said in a statement .
"He is an accomplished editor who has done a first class job on improving the performance and profile of The People having spent several years on the Daily Mirror in a senior executive role.
"Both Richard and Tina leave with our best wishes for the future and our thanks for the extremely valuable contribution they have made to the Daily Mirror and Sunday Mirror during their tenure as respective editors.'
Mr Wallace, who had been with the newspaper since 1990, had been editor of the Daily Mirror since 2004 when he succeeded Piers Morgan.
Ms Weaver had run the Sunday Mirror since 2001, having joined in 1995.
They were both among UK tabloid editors called to give evidence at the Leveson Inquiry into press ethics.
On the subject of phone hacking, Mr Wallace said he had no knowledge of the practice taking place under his editorship, but added it may have been hidden from him if had occurred previously.
In her evidence, Ms Weaver said she believed phone hacking did not take place at her newspaper, but could not guarantee it did not occur.