Mr Campbell's decision to quit the paper on Tuesday was seen by the Labour Party as a signal that its only mass-circulation supporter was about to switch allegiance.
Lord Hollick's move against Mr Montgomery came before a board meeting at the company's Holborn headquarters yesterday. Lord Hollick supported Mr Montgomery's appointment last October. The change of stance was said to have been supported by Sir Gordon Borrie, the former director-general of the Office of Fair Trading, who now chairs Labour's Commission on Social Justice. Sir Gordon was appointed a non-executive director of the company last month.
Mr Campbell's departure followed the appointment of David Seymour to supervise the newspaper's political coverage. Mr Seymour formerly worked at Today, a Rupert Murdoch-owned paper, under the editorship of Mr Montgomery.
That decision provoked censure by 150 Labour MPs in a strongly- worded Commons motion yesterday. The tabling of the motion is understood to have followed discussions between Lord Hollick and Neil Kinnock, the former party leader. Other signatories include Roy Hattersley.
Mr Montgomery promised to retain the group's Labour allegiance when he took over as chief executive. John Smith, the Labour leader, expressed concern about ther paper's future political position on Tuesday. But the motion effectively calls for his sacking.
It says: 'Noting this and other changes instigated by Mr Montgomery which jeopardise the Daily Mirror's traditionally high standards of journalism, its independence from Conservative politics, the support of many of its readers and the plurality of opinion in the British press, expresses its lack of confidence in David Montgomery.'
A rival amendment to the Commons motion, supporting Mr Seymour's appointment, was tabled by the Labour MP George Galloway, saying Mr Seymour is committed to the Labour movement.