Andrew Marr, the television presenter, is in hospital after suffering a stroke, the BBC has said in a statement.
Marr, 53, was taken ill on Tuesday and was taken to hospital, where he is responding to treatment after the stroke was diagnosed.
Ed Miliband, the Labour leader, was among the first to wish him well, tweeting: "My thoughts are with Andrew and his family. Hope he gets well soon."
The BBC said in a statement: "Andrew Marr was taken ill yesterday and taken to hospital.
"The hospital confirmed he has had a stroke. His doctors say he is responding to treatment. His family have asked for their privacy to be respected as he recovers.
"We will continue to broadcast The Andrew Marr Show and Radio 4’s Start The Week with guest presenters in his absence. His colleagues and the whole BBC wish him a speedy recovery.”
Marr, a former editor of the Independent newspaper, is known to be a fitness fanatic and takes regular exercise wherever he is filming. He is a keen runner and always abstains from alcohol during January.
In 2006 he wrote in the Telegraph about how a lifetime of distance running had left him with sore Achilles tendons, one of which suddently ruptured, requiring reconstructive surgery.
He said that for much of his life running was "partly who I was" and recalled running a marathon "more or less on the spur of the moment". He admitted, however, to "a few years of hard drinking and smoking" in his youth.
Last year he travelled the world for his series Andrew Marr's History of the World and also presented a three-part series on the life and reign of the Queen in the run-up to her Diamond Jubilee.
One close friend said they had been told Marr was still able to speak, which was a positive sign as he begins his recovery.
Marr lives in London with his wife Jackie Ashley, a political journalist at The Guardian. They have a son and two daughters.
Gordon Rayner, Telegraph.co.uk