South Africa's ex-president Thabo Mbeki claimed in an interview that Britain had urged it to topple Mugabe when a political and economic crisis escalated in the late 2000s.
But Blair's spokesman denied this had happened.
"Tony Blair has long believed that Zimbabwe would be much better off without Robert Mugabe and always argued for a tougher stance against him, but he never asked anyone to plan or take part in any such military intervention," he told AFP in London.
"Tony Blair... Was saying to the chief of the British armed forces: 'You must work out a military plan so that we can physically remove Robert Mugabe,'" Mbeki said in the interview published on November 23.
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"We knew that because we had come under the same pressure, that we need to cooperate in some scheme. It was a regime-change scheme even to the point of using military force," he added.
Mbeki's spokesman Mukoni Ratshitanga told AFP the statesman stood by his words.
Mbeki, who led South Africa from 1999 to 2008, was the head mediator between Mugabe and his arch-rival Morgan Tsvangirai after violent attacks followed disputed polls in 2008.
Mbeki had always called for a negotiated solution, resisting in particular Western interference in African affairs.
"Why does it become a British responsibility to decide who leads the people of Zimbabwe?" he told Al-Jazeera.
In November 2007 Zimbabwe put its military on high alert after retired UK army chief Lord Charles Guthrie said London had discussed invading its former colony during Tony Blair's premiership.
Blair stepped down in 2007.
That year, when Tsvangirai was assaulted and imprisoned, Foreign Office Minister Lord David Triesman told the British parliament's upper house an invasion was not on the cards.