One of Margaret Thatcher's most trusted advisers urged her not to stand for another election after she overcame a disastrous campaign to secure a third successive term in office.
Private secretary Charles Powell described the level of abuse ahead of the 1987 vote as "unbelievable" in an intimate note to the prime minister, and told her: "It's not right that you should be subjected to a further round like this time."
The letter, signed "with affection and respect" by Lord Powell and his wife Carla, has been published in full for the first time as part of the latest release of Lady Thatcher's private papers.
Lord Powell, who was shown the letter again ahead of its publication, said he had been "distressed" by the nature of the election and had sought to discourage the prime minister "from any inclination to go 'on and on'".
Two days after the end of a heavily-criticised Conservative campaign, Lord Powell offered "warmest congratulations" to Lady Thatcher on a "remarkable election victory".
He wrote: "If ever a party and a country were carried to success on the shoulders of one person, it has been over this last eight years, and the election was the reward."
But he added: "All the same I hope that you will not put yourself through it again. The level of personal abuse thrown at you during the campaign was unbelievable and must take some toll, however stoic you are outwardly.
"There comes a point when your reputation and standing as a historic figure are more important to your party, to your cause and to the country than even you yourself can be, and it's not right that you should be subjected to a further round like this time.
"I fear that, because the Left know that they cannot defeat you on substance, they will only redouble their abuse over the next few years."
He went on: "In two or three years time, you will have completed the most sweeping change this country has seen in decades and your place in history will be rivalled in this century only by Churchill. That's the time to contribute to some other area!"
Lord Powell said he had only remembered writing the private letter when extracts were published in Charles Moore's biography of Lady Thatcher.
Ahead of its public release, he said: "It's an unusual letter for a civil servant to send a Prime Minister even on a very personal basis, reflecting the size and intimacy of Number 10, especially in those days.
"I had been distressed to observe at close quarters the stress of a third election campaign and the back-biting it involved on Margaret Thatcher's health and performance and wanted to discourage her in her own interests from any inclination to go 'on and on'.""
The Margaret Thatcher Archive Trust is gradually overseeing the release of her private files through the Churchill Archive Centre at Cambridge University.
Members of the public can browse the archive by visiting www.margaretthatcher.org.