Their year-long liaison is exposed by the News of the World today, but friends had been expecting a split for some time.
Informed sources said last night that the disclosure of Mr Cook's affair could be one in a series involving Labour Cabinet ministers. Two further scandals are said to be "imminent".
The first affair, involving Clare Short, the International Development Secretary, and Jim Marshall, the Labour MP for Leicester South, was made public in the Sunday Mirror two weeks ago.
Sources claimed that the Conservative Party had instigated investigations into the private lives of senior Opposition figures in the run-up to the general election, in the hope of turning the "sleaze factor" against Labour. But the unexpected conversion of Rupert Murdoch and Lord Rothermere's tabloid titles to Tony Blair's New Labour was said to have diminished interest in "anti-Labour" scandals. "The fact that this is coming out now, months after the election and immediately after MPs have gone on a three-month summer break, tends to confirm this suspicion," the sources added. "Certainly, the timing is very convenient."
Mr Cook, 51, confirmed in a statement last night that he is leaving his 50-year-old wife Margaret, a hospital consultant. He added: "I want to make it clear that the responsibility for this is entirely mine. Margaret and I now hope to restructure our lives.
"Throughout my political career I have tried to keep my family out of the public eye. I deeply regret this will cause such public distress to them. I accept that I am a public figure but I would ask the privacy of those involved be respected at a very painful time."
The News of the World published photographs of Ms Regan letting herself into the Foreign Secretary's private flat in London. It did not say when they were taken.
A spokesman for Mr Blair said last night: "The Prime Minister is very sorry for Robin and Margaret and he feels for all concerned. He sees this as a personal tragedy for those involved which does not affect Robin Cook's capability as a truly outstanding Foreign Secretary.
"While in an ideal world, all marriages would be lasting, the truth is that these situations do arise and he hopes than Robin and Margaret can be left to rebuild their lives."
Mr Cook married Margaret in 1969, and the couple have two sons, 24 and 22. Ms Regan, who separated from her husband three years ago, has worked for Mr Cook since 1992.Reuse content