At 6.30am, two men arrived at the gates of Downing Street asking to be let in to see the Prime Minister. The policeman on the gate phoned through to Charles Powell, who was already at his desk. The two turned out to be Tory backbenchers Michael Brown and Edward Leigh.
Powell gave them coffee and explained the PM was dressing and asked them to wait. They waited and waited - in vain. They were still there when the Cabinet convened at 9am. They were put out of their misery only when the PM's political secretary, John Whittingdale, told them what they had already guessed: she was resigning.
At 7am, Cecil Parkinson was barely awake. The shrilling of the telephone put paid to that. It was Chris Chope, one of his junior ministers and a key member of the No Turning Back Group. "She's going," he said. "You've got to do something."
After Chope's phonecall, Parkinson immediately phoned No 10, only to be told that the PM was under the hair dryer and that he should phone back in 30 minutes. In desperation, he then phoned his friend of 20 years' standing, Norman Tebbit.
Tebbit had been with her until late the previous night working on her speech for the Censure debate. He told Parkinson the game was up: her mind would not be changed.
By 7.30am, Andrew Turnbull had been at his desk for an hour already. He sat there unable to concentrate. The call came.
It was the news he had expected, as the Prime Minister asked him to put in place the formal arrangements for her resignation announcement.
The next call he made was to the Palace to arrange for the formalities of an audience with the Queen.
At 8.30 every Thursday morning, it was usual for the Prime Minister to hold a short briefing in preparation for Prime Minister's Questions.
As usual, Bernard Ingham, Charles Powell and John Whittingdale were with her. It was a subdued meeting and no one was really concentrating.
The regular Thursday Cabinet meetings were a matter of routine for most of those who attended them. This one was different.
Cabinet meetings normally start at 10.30am, but this one had been brought forward so as not to clash with a memorial service for Lady Home, which was to be held later in the morning at St Margaret's Church.
Normally, the Cabinet would gather for coffee 15 minutes before the meeting; then the Prime Minister would rush into the room. That was the signal for the rest of them to take their seats around the famous oval table.
But on this morning the atmosphere was strained to say the least. Thatcher's arrival was normally the signal for everyone to file into the room and take their places, but it seemed there was a delay.
The awkward silence continued for an unbearable 10 minutes. At 9.10, the Cabinet filed in. The PM was in her usual chair, halfway along the table in front of the fireplace.
For the first time in living memory, the woman who had dominated her Cabinet for 11 years seemed powerless. Slowly, Thatcher opened her handbag and pulled out a creased piece of paper. She read in a slow, halting, emotional manner:
"Having consulted widely among my colleagues, I have concluded that the unity of the party and the prospects of victory in a General Election would be better served if I stood down to enable Cabinet colleagues to enter the ballot for the leadership. I should like to thank all those in the Cabinet and outside who have given me such dedicated support."
She faltered several times and broke down sobbing. Halfway through the statement she was so upset that Cecil Parkinson, already on a light fuse, shouted to the Lord Chancellor, who was sitting to her left: "For Christ's sake, you read it, James."
Lord Mackay briefly put his arm round her shoulder and said gently, "Let me read it, Prime Minister." This brief interjection broke the unbearable tension and allowed the Prime Minister a few moments to gather herself. She stiffened both in resolve and body language and said, "No! I can read it myself."
The Lord Chancellor then read out a short tribute to the Prime Minister. She listened, eyes glistening and red, and broke down again. She regained composure and told the Cabinet it must unite behind a candidate to beat Michael Heseltine. "We must protect what we believe in," she flashed.
The meeting broke for 10 minutes and coffee was served. Courtesy calls were made to the other party leaders and the Speaker. A formal statement was issued by the Downing Street Press office.
By the close of the meeting, the Prime Minister was close to tears again.
No one was keen to be the first to leave, although Douglas Hurd didn't hang around long. Cecil Parkinson's most vivid memory was when somebody - allegedly Kenneth Clarke - said: "We are going to pin regicide on Heseltine." For a moment the PM looked puzzled and issued a devastating reply: "Oh no, it wasn't Heseltine; it was the Cabinet."
As the Cabinet left Downing Street, Kenneth Baker made a short statement outside No 10: "This is a typically brave and selfless decision by the Prime Minister. Once again Margaret Thatcher has put her country and the party's interests before personal considerations. She is an outstanding leader, not only of our country but also of the world. I do not believe we will see her like again."
While Denis Thatcher attended the memorial service for Lady Home, the Prime Minister was driven to Buckingham Palace, informing the Queen in person of her decision. It was not a long audience.
The Prime Minister was well aware she had the speech of her life to make in the Commons in just a few hours' time. It was to be an occasion she - and the country - would have cause to remember for many years to come.