Theresa May has told her critics that getting rid of her as PM would not make delivering Brexit any easier.
Mrs May, who has been facing calls for a no-confidence vote from some of her MPs since her draft plan for leaving the EU was published on Wednesday, said there was a "critical" week ahead.
Talks on the UK's future relationship with the EU were continuing ahead of an expected EU summit next week, she said.
Jeremy Corbyn said Labour could get a better deal in time for Brexit.
There has been widespread criticism of the draft withdrawal agreement reached between the UK and the EU, which is set be signed off at a special summit next week.
There is also doubt over whether it can win the approval of the House of Commons, with opposition parties - including Northern Ireland's Democratic Unionist Party - opposing it.
Some cabinet ministers have resigned and others are believed to be still trying to change its wording.
In other developments:
"Politics is a tough business and I've been in it for a long time," she said, adding that the next seven days "are going to be critical" for the future of the UK.
Asked whether Sir Graham Brady - chairman of the backbench 1922 committee - had received the 48 letters needed to trigger a confidence vote in her leadership, she replied: "As far as I know, no - it has not."
And in a warning to those pushing for a change of leader, she said: "It is not going to make the negotiations any easier and it won't change the parliamentary arithmetic."