Baroness Thatcher has pulled out of a visit to Downing Street to mark her 85th birthday because she is suffering from flu.
The former prime minister had been due to attend an event hosted by David Cameron with about 150 friends and colleagues on Thursday evening.
At Baroness Thatcher's request, it went ahead in her absence, a government spokesman said.
Baroness Thatcher's birthday was on Wednesday.
She was taken ill at home on Thursday.
Guests, who included previous members of her cabinets such as Lord Tebbit and Lord Howe, will be invited to return to Downing Street to celebrate once she has recovered.
In a statement read out by Mr Cameron at Thursday night's event, Baroness Thatcher said: "After all the trouble that everyone has been to, I am so disappointed not to be with you this evening.
"I want to thank the prime minister and everyone present for your understanding.
"I hope that you will appreciate that on this particular occasion I have had to accept that the Lady is not for returning. Please, please enjoy yourselves."
Mr Cameron invited Baroness Thatcher, Prime Minister from 1979 to 1990, to Number 10 soon after he took over the premiership from Labour's Gordon Brown in May.
He told the Conservative conference in Birmingham last week that she had been "the greatest peacetime prime minister of the 20th century".
On the advice of doctors, Lady Thatcher, who has previously suffered minor strokes, rarely speaks in public, but still attends public functions.
In March 2008 she was taken ill during a dinner in Westminster and spent the night in hospital as a precaution, bit was later given a clean bill of health.