Brexit minister Dominic Raab has said the UK should reject an extended transition period unless the EU abandons its demand for an Irish backstop.
Theresa May's suggestion that the UK could remain attached to the European Union after December 2020 could only be an "alternative" to the backstop suggestion, Raab wrote in the Telegraph.
The backstop is an the EU's insurance policy to make sure there is no hard border between Northern Ireland and Ireland if a trading deal is not secured in time.
However, the European Union's proposal would involve Northern Ireland temporarily remaining in the EU's custom union.
The news comes the day after thousands of protestors marched through central London to demand a second referendum on Brexit.