Gerry Adams was in hospital during the week, a Sinn Féin spokesperson has confirmed.
He has been undergoing tests following a muscle injury he sustained while cycling.
He had been due to travel to London on Wednesday to make a speech commemorating former London Mayor Ken Livingstone's late advisor Redmond O'Neill.
His doctor advised him not to travel due to the pain from the injury.
A spokesman did not provide specific details, saying "he wrenched something".
Mr Adams wrote in his blog: "I was to give the inaugural Redmond O’Neill lecture in London on Wednesday night. All of the arrangements were made and the lecture was being held in Bolivar Hall, courtesy of the Venezuelan Embassy.
“Unfortunately I had to pull out at the last minute. I hate doing that. But thankfully Pearse Doherty stepped into the breach and with minutes to spare managed to catch a slightly later flight."