If the European Council decides to agree an extension of the period in Article 50(3) of the Treaty on European Union ending at 11.00 pm on 31 October 2019 to the period ending at 11.00pm on 31 January 2020, the Prime Minister must, immediately after such a decision is made, notify the President of the European Council that the United Kingdom agrees to the proposed extension.

If the European Council decides to agree an extension of the period in Article 50(3) of the Treaty on European Union ending at 11.00pm on 31 October 2019, but to a date other than 11.00pm on 31 January 2020, the Prime Minister must, within a period of two days beginning with the end of the day on which the European Council’s decision is made, or before the end of 30 October 2019, whichever is sooner, notify the President of the European Council that the United Kingdom agrees to the proposed extension.

But subsection (2) does not apply if the House of Commons has decided not to pass a motion moved by a Minister of the Crown within a period of two calendar days beginning with the end of the day on which the European Council’s decision is made or before the end of 30 October 2019, whichever is sooner, in the following form—

Nothing in this section shall prevent the Prime Minister from agreeing to an extension of the period specified in Article 50(3) of the Treaty on European Union otherwise than in accordance with this section.