Ex-cabinet minister Mo Mowlam is widely popular
A spokeswoman at King's College Hospital said the 55-year-old's condition was still "critical but stable" on Friday.
She would not give more details of Ms Mowlam's illness or say whether it was connected to her previous brain tumour.
Ms Mowlam, Labour MP for Redcar between 1987-2001, oversaw the talks which led to the 1998 Good Friday Agreement.
She was admitted to the hospital last weekend.
Humour
Democratic Unionist Party leader, Ian Paisley, on Thursday said Ms Mowlam was no fan of his party but he was praying for her.
He said: "I, personally in my prayers, I've prayed for her, and I hope she'll get over this because we don't wish her any ill whatsoever.
"But that doesn't mean that we haven't had political differences with her."
As a minister Ms Mowlam, who is married to merchant banker turned painter Jon Norton, was a popular and charismatic figure, known for her outspoken views and sense of humour.
In 1998, as Northern Ireland secretary, she went inside the Maze Prison when it became clear that the peace process would only succeed with the backing of the prisoners. The loyalist UDA/UFF prisoners had previously withdrawn their support for the process.
She spoke to the prisoners face-to-face for 60 minutes, and two hours later the paramilitaries' political representatives announced they were being allowed to rejoin the talks.
She told reporters at the time: "I didn't negotiate, I didn't do a deal. If you want progress, you ain't going to get it if you don't have talks."
In 1999 she was replaced as Northern Ireland secretary by Peter Mandelson, and became Tony Blair's cabinet "enforcer". She subsequently stood down as an MP and in recent years has pursued a career in the media.
Former Labour Party MP and Secretary of State for Northern Ireland Mo Mowlam, 55, is reported to be critically ill at King's College Hospital, London.

According to a King's College Hospital spokeswoman, Ms. Mowlam is "critical but stable," and added that her family did not want further details disclosed.  

Ms. Mowlam survived a brain tumour in the 1990s, which saw her lose her hair, forcing her to wear a wig. 

Many state that her charismatic personality played an important role in the Northern Ireland peace talks, leading to the Good Friday Agreement in 1998 and an IRA ceasefire.