Boris Johnson was the kind of person most people thought would just "bulldoze through", said former London Assembly member Baroness Jenny Jones.
The Green peer frequently clashed with Boris Johnson when he was London Mayor. But she said she was upset by news of his worsening condition.
On Tuesday Downing Street said Boris Johnson was "stable" after spending the night in intensive care.
The Prime Minister's official spokesman said he had been receiving "standard oxygen treatment" but had not required any other assistance in breathing.
Mr Johnson was transferred to the intensive care unit at St Thomas' Hospital, London, on Monday evening, after his condition deteriorated.
His spokesman said the move was a "precautionary step" in case he needed to be put on a ventilator.