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The claim: Boris Johnson said: "Were we obliged to the stay in the EU, we would need a bigger bus, because the figure would go up and I think it will be rising [to] £400m gross."

Reality Check verdict: The UK's gross contribution to the EU budget is due to rise to £406m a week by 2021. But this figure does not include the UK rebate - a discount applied before any money is sent to Brussels. In 2021, once the rebate is deducted, the weekly contribution would be £325m.

Boris Johnson has talked about the cost to the UK of staying in the EU if Brexit is delayed beyond 31 October.

In a BBC Radio 4 Today programme interview, the prime minister was challenged over his figures. The issue of the claim about £350m weekly payments to the EU, written on the side of a bus in the 2016 referendum campaign and criticised as "potentially misleading", was also raised.

Mr Johnson responded that if the UK was "obliged" to stay in the EU, then "we'd need a bigger bus" because the figure would rise, he claimed, to £400m a week gross.

So, is he right?

The Treasury has published a table based on data from the Office for Budget Responsibility - an independent organisation that monitors government spending - setting out how much money the UK would be expected to contribute to the EU in the years ahead.

Adjusting the above numbers to a weekly basis, the OBR's projection shows the UK's gross contribution would rise to £406m a week in 2020-21.

But this figure does not factor in several key things which - when included - result in a very different net figure.

First, the UK rebate.

This is a discount to the UK's budget contributions, applied before any money is sent to Brussels.

It was negotiated by former Prime Minister Margaret Thatcher and introduced in 1985.

While it remains a member of the EU, the UK could lose its rebate only if all members (including the UK itself) agreed to it.

For 2020-21, the rebate is expected to be £4.2bn. Once that amount is knocked off, the UK's bill is £325m a week.

There's also the money the UK receives from the EU (public sector receipts) for payments to farmers, development and other projects.

This money is allocated by the EU and distributed by the UK government.

For 2020-21, it is expected to total £5.9bn. If this was factored in as well this would leave a net figure of just over £210m a week.

There are further payments the EU makes directly to the UK private sector, such as grants to universities. They do not form part of the above calculation but the ONS says the UK receives on average around £1.1 billion a year, or £21 million a week, in those payments.

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