Baroness Thatcher had an hour of private talks in Downing Street with Prime Minister Gordon Brown - strengthening the Premier's claim to be a leader reaching across party divides.
Lady Thatcher's visit lasted two hours and 12 minutes in total as she was also given a guided tour of No 10, and took tea with Mr Brown, his wife Sarah and Chancellor Alistair Darling's wife Margaret in her old flat.
The visit will inevitably be seen as another move by Mr Brown to try to foment discontent with Tory leader David Cameron among Conservative ranks.
Downing Street had no official comment after the talks ended. But sources earlier revealed that Mr Brown's invitation to tour No 10 arose from an exchange of letters in the first few days after he succeeded Tony Blair on June 27.
At a press conference earlier this month, Mr Brown also described Lady Thatcher as someone who had seen the need for change in Britain, adding: "I also admire the fact that she is a conviction politician. I am a conviction politician like her."
Mr Brown and Lady Thatcher had their private political discussions in the Prime Minister's study, still known by some staff as the Thatcher Room since it was where she too worked late into the night on policy issues.
About 10 to 15 members of staff who were present when she left office were lined up to meet Lady Thatcher - mainly custodians of the building and clerical workers.
The visit brought criticism from Paul Kenny, general secretary of the GMB union, who said: "This is a huge political mistake which will cost the Labour Party credibility with their core voters and communities up and down the country that still bear the scars of the damage she caused.
"Mrs Thatcher was put out of Downing Street by the Tories themselves. It is unbelievable that she should be invited back by a Labour prime minister. GMB members will simply not understand this."
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Tony Woodley, joint general secretary of the Unite union, said: "Political tactics are all very well but millions of people will remember the 1980s, the devastation of the mining communities and three million people on the dole and they will find the sight of any Labour party member, let alone the Prime Minister, welcoming Mrs Thatcher deeply offensive."
