No 10 has denied that Gordon Brown "exploited" Lady Thatcher for political ends by inviting her to Downing Street.
Lady Thatcher was greeted with a handshake
Lady Thatcher returned to her former home on Thursday for private talks with Mr Brown and a tour of the building.
Mr Brown's official spokesman said the meeting was "not unusual" and he would be "happy" to meet any former PM.
But the meeting was criticised by some Conservatives, with one MP accusing Mr Brown of exploiting the "frail" 81-year-old for his own ends.
Lady Thatcher spent more than two hours in Downing Street, 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.
But Conservative MP Rob Wilson, writing on the influential ConservativeHome website, said the meeting made him feel "uneasy".
"Yesterday's visit to Downing Street wasn't really about Margaret Thatcher, it was really about Gordon Brown.
"My feeling of delight for the Baroness going back to the scene of her former glories was tempered by an uneasy feeling about the way she was apparently being exploited by the prime minister.
"Baroness Thatcher is 81, she is elderly, she is lonely, she is frail and she has difficulty with her memory. Those closest to her say that her grasp on daily life is 'some days better than others'."
The Reading East MP said Lady Thatcher would "naturally have been flattered by an invitation to Downing Street and would have enjoyed being back in the thick of the political action".
But he asked whether it was "right" that a serving prime minister "should exploit a situation like this for his own political ends?".
"Was this another carefully-choreographed Brown 'dog-whistle' to Conservative-minded electors that it's 'safe' to vote for him?," added Mr Wilson.
Downing Street denied the prime minister had "exploited" Lady Thatcher, who led the country between 1979 and 1990.
"Lady Thatcher is a distinguished former prime minister. It is not unusual for a previous prime minister to meet a current prime minister," said Gordon Brown's official spokesman.
"I am sure he would be very happy to meet any previous prime minister."
He said the visit had been arranged after an exchange of letters between Mr Brown and Lady Thatcher shortly after he became prime minister in June.
Baroness Thatcher's spokesman said she had been "fully aware" of the political ramifications of the meeting.
"I don't think these comments [by Mr Wilson] are helpful. To suggest that she was not aware of the political implications of it would be wrong.
"It was just one prime minister courteously inviting another prime minister."
Mr Wilson added to his attack on Conservativehome, saying the handling of the visit exposed Mr Brown's "deep-rooted character flaws".
"There will be questions about Brown's moral compass and whether he used Baroness Thatcher in a shallow, self-serving and unscrupulous way," he wrote.