Parliament normally sits for one year, but officials said late Saturday night more time will be needed.
House of Common leader Andrea Leadsom said Parliament will need "the maximum amount of time to scrutinise these bills" by holding a two-year session.
The legislation is expected to include the Great Repeal Bill to convert existing European Union law into United Kingdom statutes.
Prime Minister Theresa May has been struggling to put together a working government since the Conservative Party lost its majority in the June 8 election. She is seeking an arrangement with the Democratic Unionist Party in Northern Ireland.
Formal Brexit talks with the European Union begin Monday.