Section 1 of the European Union (Withdrawal) Act 2019

The honest answer is that we all know that 30 June is not a particularly realistic proposition and that the Prime Minister was forced to propose that date more for reasons of party management. She has, in a sense, contracted out the decision to the EU. We would expect the Government to accept any reasonable extension that goes beyond 30 June, with the proviso that if this House approved and ratified a withdrawal agreement we would exit at that point.