We checked Boris Johnson's claim that a Brexit extension will cost £1bn a month – and yes, it's wrong

In Boris Johnson’s first prime minister’s questions yesterday he claimed it would cost a net £1bn per month for Britain to stay in the EU beyond 31 October.

The claim echoes similar comments from foreign secretary Dominic Raab. Speaking on BBC Radio 4 earlier this week, he also said that the bill put forward by Hilary Benn and others – intended to rule out a no deal Brexit unless parliament consents – would cost British taxpayers £1bn a month. That’s based on the fact that we would be paying into the EU budget for longer.

This is an example of how a claim can be misleading without necessarily being incorrect. It needs more context.

The figure is based on a comparison with a no deal scenario – something that the government itself has said it doesn’t want. It relies on the assumption that under no deal, the UK won’t pay the “divorce bill” agreed with the EU. And it completely ignores a pretty fundamental point: that no deal itself would have significant costs.

The “divorce bill” agreed by Theresa May’s government already includes the UK’s normal payments into the EU budget until December 2020. The legislation currently before parliament itself proposes an extension only until January 2020.

As such, any payments into the EU budget following an extension during this period would reduce the final “divorce bill” paid upon leaving the EU by the same amount. And so, leaving with a deal in January 2020 wouldn’t cost more in EU budget contributions than leaving with a deal in October 2019 would.

This has already happened once: the original settlement was around £39 bn, but this is now down to £33bn, largely as a result of extending Brexit from March to October this year.

The government could of course plan to try and renegotiate this financial settlement before 31 October, but we aren’t aware of any indication so far that they hope to do this. The government position is that it still wants to leave with a deal, and any renegotiation efforts are focused on the question of the Irish backstop.

If Johnson is comparing the cost of any extension to a no-deal exit at the end of October, then his figure of £1bn for monthly EU contributions is in the right ballpark. However, as BBC Reality Check has pointed out, this doesn’t factor in payments received from the EU, which would reduce the amount by about a quarter.

It’s also not entirely clear that leaving with no deal would mean we could avoid paying the “divorce bill”, something which Boris Johnson has said several times would be his plan in a no-deal scenario.

When Full Fact looked at this question earlier this year, we found that under international law it’s not clearly set out that the UK has to pay anything once it has left the EU. However, the EU would be within its rights to take the case to the International Court of Justice on the grounds of the UK’s repeated commitments to pay, and the outcome of such a case would be hard to predict.

Like a lot of what’s said by all sides in the Brexit debate, context is everything. While not necessarily wrong to compare the potential cost of an extension to a no-deal exit, it’s misleading to do so given that the government’s stated intent is still to leave with a deal.

And if you’re going to make such a comparison, then you do need to actually compare the costs of both things – you need to factor in the fact that no deal itself will have both direct costs, and a broader economic impact. Most studies predict that the negative economic effects of leaving the EU will exceed the benefits of not having to pay the EU membership fee.

Tom Phillips is editor of Full Fact, the UK's independent fact checking charity