Theresa May is setting out the details of a series of compromises designed to try and win the support of Labour MPs for her Brexit plan.
The cabinet earlier agreed the idea of a temporary customs relationship until the next general election, and measures on the environment and workers' rights.
These will be included in the Withdrawal Agreement Bill, to be put to a vote in the Commons in early June.
The SNP and some Tory Brexiteers have already said they will vote against.
The PM briefed MPs and ministers on the contents of the speech - entitled "A new Brexit deal - seeking common ground in Parliament" - beforehand.