Lady Thatcher was greeted with a handshake
Former prime minister Baroness Thatcher returned to Downing Street earlier for an hour of private talks with Gordon Brown and a tour of Number 10.
Lady Thatcher, 81, spent more than two hours at her former home, where she was greeted with a handshake from the PM.
Mr Brown recently surprised many by praising Labour's long-time political foe as a "conviction politician".
The Tories said they were "relaxed" about the meeting, pointing out that she had previously met Tony Blair.
The visit followed an exchange of letters between Mr Brown and Lady Thatcher, who led the country from 1979 to 1990, shortly after he became prime minister in June.
The two spent an hour in private talks in the formal study - the Thatcher Room - before taking a tour of the building where prime ministers live as well as work when in power.
Lady Thatcher also had tea with Sarah Brown and the chancellor's wife, Maggie Darling, in the flat above No 10 - which she had refurbished while she was in power.
She left clutching a bouquet of flowers, without commenting to reporters, but thanked her hosts as she was helped to her car.
The visit, though unusual, is not unprecedented and does not mean that Lady Thatcher is backing Mr Brown as prime minister - shortly after Tony Blair took power in 1997, he also extended an invitation to Baroness Thatcher.
Downing Street sources said one of Mr Brown's "biggest and earliest political memories" after becoming an MP was an invitation for him to visit the then prime minister in her Commons office in 1983 to discuss a speech he had made.
But Paul Kenny, general secretary of the GMB union, said the visit was "a huge political mistake" which would damage Labour's credibility with its core voters.
"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," he said.
The visit comes on the day it emerged that the advertising agency credited with helping secure the 1979 election victory for the then Mrs Thatcher - Saatchi and Saatchi - has been given the Labour general election advertising account.
Mr Brown's comments last week praising Lady Thatcher were taken as him suggesting that he - and not current Tory leader David Cameron - was a conviction politician like her.
But Lady Thatcher's former private secretary, the Conservative MP John Whittingdale, said she would have been touched by the invitation and said it was a "nice gesture" by Mr Brown.
He said: "She has an immense respect for the office of prime minister and if the prime minister invites you to come to tea - you don't say no."
He added that he knew Lady Thatcher and Mr Cameron "see each other occasionally".
A Conservative spokesman said they were "relaxed" about the meeting, adding: "She met, as a former prime minister, with Tony Blair. Now she's meeting Gordon Brown."
Shadow defence secretary Liam Fox said he hoped Mr Brown would be able "to learn a thing or two from a very successful Conservative prime minister".
A spokesman for Lady Thatcher later said she had visited each of her successors as prime minister, including John Major and Mr Blair, but would not comment on the content of her talks with Mr Brown.