Sir Edward Heath was in hospital last night after suffering a suspected blood clot on his lung while on holiday.
The former Conservative prime minister, who is 87, was flown back from Austria yesterday after having been taken ill with a suspected pulmonary embolism. Upon his return, Sir Edward, a non-smoker who rarely drinks, was transferred to the private King Edward VII hospital in central London for further treatment.
A spokesman for Sir Edward said: "Following treatment in hospital in Salzburg for a minor stomach upset, a small clot on the lung was discovered. He is now receiving further treatment for this condition at the King Edward VII hospital."
Sir Edward served as Prime Minister from 1970 to 1974 and is best known for successfully negotiating Britain's entry into the forerunner to the European Union, the European Economic Community. He stepped down from the House of Commons at the 2001 general election after more than 50 years as an MP. He was knighted in April 1992, but said he had no desire to take up the seat in the House of Lords customarily offered to former prime ministers.
Last night, Iain Duncan Smith, the Tory party leader, said: "Naturally, we all wish him a very speedy recovery. The thoughts of the entire Conservative Party are with Sir Edward at this time."
Derek Conway, who succeeded Sir Edward as Conservative MP for Old Bexley and Sidcup, described how the former premier fell ill during his "annual pilgrimage" to the Salzburg festival in Austria. "We will all be hoping that this is a scare and nothing more that a scare," Mr Conway said.
"He is held in very high regard in Old Bexley and Sidcup and a lot of people are wishing him a very speedy recovery."
The MP described how he had last seen Sir Edward during a dinner held in his honour by Michael Martin, the House of Commons Speaker, only six weeks ago.
Despite depending heavily on the use of a wheelchair to rest his legs, Sir Edward appeared to be in robust health at the time, according to Mr Conway. "He is not the kind of man who sits around fretting about years gone by, but is still out and about and is in full control of his faculties," he said.
"Physically some things are a bit of a trial but mentally he is absolutely cogent."