It’s not true to say a Brexit extension would cost £1 billion

Wrong. An extension of a few months would cost no more than leaving with a deal in October 2019, which is still the government’s publicly stated policy.

“I think that it would be a mistake to keep the UK bound in beyond the time that the people want to come out… at a cost of £1 billion a month.” Boris Johnson, BBC Radio 4 Today, 1 October 2019 (2 h 21 m)

This morning, the Prime Minister repeated his inaccurate claim that extending the date of Brexit beyond 31 October would cost “£1 billion a month”.

This is not true. The Prime Minister says he wants to leave on 31 October with a deal. If he does, the UK will be in a transition period to the end of December 2020, during which it will pay into the EU budget anyway.

So extending the date of Brexit into 2020 won’t, by itself, cost us any more in budget contributions than leaving with a deal in October this year would. Those payments just get taken off the final “divorce bill” when we leave.

The government has given no indication that it plans to renegotiate either the financial settlement part of the deal—the divorce bill—or the transition period in the month remaining. That situation hasn’t changed since we first fact checked this a month ago.

And if you’re not planning to renegotiate the divorce bill or the transition, then the only way you can claim extending would cost us £1 billion more is if you’re comparing it to leaving with no deal, and refusing to pay the divorce bill.

The Prime Minister has said that, in the event of no deal, he does plan to not pay the financial settlement (the legal situation of that would be uncertain). But he also repeated in his Today interview that his plan is to leave with a deal.

The EU has also made clear that it will make the divorce bill a condition of any future trade deal.

You can see evidence that a short extension wouldn’t cost us more from the fact this has already happened once. Because the UK has stayed a full EU member longer than expected, we have paid budget contributions since 29 March. But all that means is that the divorce bill has gone down by the same amount: the original divorce bill was around £39 billion in March, but this has now gone down to £33 billion. It does not in itself make any difference to how much we pay.

If the UK had a deal but extended the transition to 2021 or 2022 (which is possible under the current agreement if both sides agree, but is not something the Prime Minister has said he wants to do), then the UK would pay more, as it would continue contributing to the EU budget for those extra years. But we don’t know how much that would be, as the EU hasn’t yet set its budget for then.

Mr Johnson’s government has publicly said it is committed to getting a deal with the EU, and hasn’t said it wants to renegotiate the divorce bill or the transition. That’s what makes this claim wrong.

If the government publicly said its policy was to go for no deal, then it could argue that it would try to avoid paying the divorce bill. If they then succeeded in avoiding that bill, this claim might be in the right ballpark (although still not quite right, as BBC Reality Check have pointed out).

But as it stands, the Prime Minister asserted two contradictory things in the same interview. You can say your plan is to leave with a deal, or you can say an extension will cost £1 billion a month, but you can't say both.