While his doctors foreshadowed courses of chemotherapy and radiation treatment, the tumour was identified as a glioma, an aggressive form of cancer.

The US Society for Neuroscience said that gliomas are very damaging because they spread quickly within the brain. More than half patients diagnosed with a glioma die within 18 months.

The diagnosis was dominating American newscasts overnight and prompted widespread tributes to the long-serving "Lion of the Senate".

President George Bush said he was deeply saddened and that he would pray for Senator Kennedy.

"Ted Kennedy is a man of tremendous courage, remarkable strength, and powerful spirit. Our thoughts are with Senator Kennedy and his family during this difficult period," Mr Bush said.

One of those who hopes to succeed the President, Barack Obama, said he was shattered by the news.

Senator Obama said that Senator Kennedy was a fighter and a great senator.

"I would not be sitting here as a presidential candidate had it not been for some of the battles that Ted Kennedy has fought," Senator Obama told CNN television.

"Not only is he a personal friend, but he is somebody who battled for voting rights and civil rights when I was a child. I stand on his shoulders."

Senator Kennedy, 76, has been a dominating figure in the US Senate over seven full terms. His endorsement of Senator Obama for the Democratic presidential nomination this year was a key moment in Senator Obama's contest with New York Senator Hillary Clinton.

Senator Kennedy was the youngest brother of president John F. Kennedy, who was assassinated in 1963. A second brother, Bobby Kennedy, was assassinated in 1968 while running for the presidency.

Senator Kennedy is being treated in Boston's Massachusetts General Hospital. Doctors said in a statement that he was in good spirits and that his course of treatment had not been finalised.

"Preliminary results from a biopsy of the brain identified the cause of the seizure as a malignant glioma in the left parietal lobe," they said.