Senator Edward Kennedy, the sole surviving son of America’s most fabled political dynasty, is suffering from a malignant brain tumour.

Doctors treating him after he suffered a seizure at the weekend announced yesterday that a biopsy had shown a form of cancer known as a glioma in the left side of his brain. The prognosis appears gloomy with survival rates ranging from less than 12 months to about five years.

The 76-year-old Democratic senator left hospital in Boston today. He had been treated there since Saturday, after he was airlifted from the family’s Cape Cod compound. His wife, Victoria, and children have been with him each day but have made no public comment.

The diagnosis yesterday came after initial fears that he had suffered a stroke were dispelled and the senator was reported to be "driving nurses crazy", watching a Red Sox baseball game and joking with hospital staff.

But yesterday, Dr Larry Ronan issued a fresh statement revealing the preliminary results of a biopsy. “The usual course of treatment includes combinations of various forms of radiation and chemotherapy,” he said, before adding that "further testing and analysis" was needed to determine the best course for the senator.

Washington’s world stopped turning yesterday as politicians from all sides held their breath and prayed for one of the genuine titans on Capitol Hill. "I am so deeply saddened I have lost the words," said long-serving Republican Senator John Warner.

The White House said that President Bush was “deeply saddened” by the news. John Kerry, his fellow Massachusetts Senator and a close personal friend, described Mr Kennedy as a “living legend”.

Harry Reid, the Senate Majority leader, said: “He is a model of public service and truly an American icon. Anyone who knows Ted Kennedy knows he is a fighter and he has risen to every challenge he has faced — and he has had plenty. We’re confident that he will rise to this one too.”

John McCain, the Republican nominee-elect who has had his own brush with cancer, said that Mr Kennedy was the “single most effective member of the Senate”. He added: “Our thoughts and prayers go out to his family and to him.”

Mr Kennedy, the second-longest serving member of the Senate was first elected in 1962, taking over the seat vacated by his brother John in winning the presidency.

But death and trauma have never been far from a family that has held much of America in its thrall for half a century. The eldest brother, Joseph, was killed in the Second World War, President Kennedy was assassinated in 1963 and Robert Kennedy was shot dead in 1968 during his own White House campaign.

Mr Kennedy, who survived a 1964 plane crash in which one of his aides died, saw his career blighted by an incident in 1969, when Mary Jo Kopechne, a passenger in his car, was killed after he drove off a bridge at Chappaquiddick. Mr Kennedy was given a two-month suspended jail sentence for the incident which contributed to his defeat when he ran against President Carter in 1980.

His son, Congressman Patrick Kennedy, is recovering from addiction problems and last year was arrested for crashing his car into a police barrier near Capitol Hill.

But through it all, the senator has remained a tireless champion of progressive causes from civil rights to opposing the war in Iraq. Earlier this year he gave dramatic backing to Barack Obama. saying that the Democratic presidential candidate can inspire America just as his brothers had once done.

He has more than four years of his current Senate term left to serve and recently signed a $8 million (£4 million) deal for his memoirs.

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