The Department for Exiting the European Union (DExEU) will be closed down on 31 January – Brexit deadline day.
The government department, led by Brexit Secretary Stephen Barclay, oversaw negotiations between the UK and EU on the exit and future relationship.
But Boris Johnson has negotiated a deal, which is expected to pass now he has a 80-strong Commons majority.
Staff were told about the closure on Thursday, but the PM’s spokesman said they would help them find jobs.
A statement from No 10 said: “The Department for Exiting the European Union will be wound up once the UK leaves the EU on the 31 January.
“DExEU staff have been spoken to today. We are very grateful for all their work and we will help everyone to find new roles.”