Broadcaster Andrew Marr is recovering in hospital after suffering a stroke, the BBC confirmed.
The 53-year-old journalist and television presenter was taken ill on Tuesday, the corporation said.
The BBC said in a statement: "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 is the corporation's former political editor.
He began his career as a newspaper reporter, becoming the editor of The Independent before turning to television.
He has also presented a number of history programmes along with his politics show and has had five books published.
In May 2011 it emerged that he had taken out a super-injunction to prevent the reporting of an affair.
He won a High Court order in January 2008 to silence the press following his extra-marital affair with another national newspaper reporter.
He admitted he had taken the step as he said he felt "uneasy" about it as a journalist.
The woman with whom he had the affair is a political journalist who has a daughter. Since their fling, some nine years ago, her name has appeared widely online.
The BBC said that James Landale will present The Andrew Marr Show this Sunday.
Many of Marr's colleagues in the media took to Twitter to express their sympathy.
Fellow BBC politics presenter Andrew Neil wrote: "Very distressed to hear news about Andrew Marr. Best wishes for full and speedy recovery."
Guardian columnist Polly Toynbee tweeted: "Andrew Marr, renaissance man, polymath, wise commentator, painter, runner, brilliant cook – and ace editor. Get well soon. Needed in public life."
Former Tory MP Louise Mensch said: "Joining in the chorus of well-wishers for Andrew #Marr. Hope he has a speedy and full recovery."
Around 150,000 people have a stroke each year in the UK with about a quarter of these in people of working age, according to the Stroke Association.
Its director of communications, Joe Korner, said: "We are deeply saddened to hear about Andrew Marr's stroke and our thoughts are with him and his family at this hard time.
"A stroke happens in an instant but the effects can often last a lifetime.
"However with the right care and support it is possible to make a recovery and return to a life after stroke."