The claim was later dismissed as 'patent nonsense' by Sir Patrick Mayhew, Secretary of State for Northern Ireland.
Meanwhile, setting out his position in a speech in advance of the summit, Mr Reynolds emphasised that his government saw cross-border institutional links as essential for any new agreement for Northern Ireland. He added, on Irish radio, that there was a 'gap to be bridged' at Chequers over the links, though not a substantial one.
Mr McGuinness, speaking in London for the first time since the lifting on Friday of an exclusion order banning him from mainland Britain, said he had been given the assurance by a government representative speaking with the 'full authority of the British Prime Minister'.
The assertion came as the Dublin- based Sunday Tribune newspaper claimed that the IRA leadership would review the ceasefire in March to judge whether the content and outcome of exploratory talks between Sinn Fein and British representatives, promised before the end of the year by Mr Major on Friday, warranted its continuance.
A resumption of violence would almost certainly see the end of the current Sinn Fein leadership, and Mr McGuinness's remarks were seen in Whitehall as largely for the consumption of a remaining hardcore of IRA and Sinn Fein members who are sceptical about the peace process.
Interviewed on the BBC 1 programme On the Record, Mr McGuinness said: 'In March of last year I did have a meeting with a representative of the British government who said to us that the eventual outcome of all that Britain was trying to do would be that the island would be as one.'
He claimed the Government had already accepted that there could be no internal political settlement for Northern Ireland.
Mr Major's speech in Belfast had significantly adopted the Republican terminology of moving towards a 'just and lasting peace', Mr McGuinness said, adding: 'I think the British government are slowly but surely disengaging from Ireland.'
But Sir Patrick said in a statement: 'No British government representative has ever been or will ever be authorised to say that we support a united Ireland unless it is by the consent of the people of Northern Ireland . . .
So long as it is their wish to remain in the United Kingdom, the British government will uphold that wish.'
While admitting he was 'sceptical' of private assurances from the Government, Mr McGuinness also dismissed suggestions by James Molyneaux, leader of the Ulster Unionists, that there could be a new Northern Ireland assembly.
'If he thinks the SDLP and the Dublin government and ourselves will support assembly elections at the beginning of next year, I think James Molyneaux is living in cloud cuckoo land,' he said.
Mr Reynolds yesterday highlighted his government's insistence on cross-border institutional links in a speech to members of his Fianna Fail party at the annual ceremony at the grave of the 18th-century Irish republican Wolfe Tone in Co Kildare.
He said: 'If nationalists are being asked at present to accept the reality and the consequences of the present majority wish of a continuation of the Union between Northern Ireland and Britain, their desire for meaningful institutional links with executive functions must also be accommodated.'
Citing a call by American industrialists for a harmonised tax regime north and south of the border, he said: 'Independent of their political justification, such institutions must also address some of the key interests common to north and south on a pragmatic basis.'