Lord Bannside, better known as the Rev Ian Paisley, is in intensive care in Belfast with heart problems.
Aged 85, he has been prominent in Northern Ireland politics for more than half a century.
Paisley, a former Belfast First Minister and head of the Democratic Unionist Party, was admitted on Sunday.
A statement on behalf of his wife, Baroness Paisley, said she "requests that the family's privacy be respected at this difficult time".
In recent years he made a dramatic turnaround, abandoning decades as a hardline rejectionist opposed to compromise to go into political partnership with Sinn Fein. In particular he formed a warm personal relationship with former IRA leader Martin McGuinness, which helped transform the tone of Belfast politics.