The UK's divorce bill with the EU could be much higher than the £35 billion to £39 billion estimated by the Treasury, it has emerged.

The National Audit Office, the official spending watchdog, said the Treasury's estimate was "a reasonable calculation".

But it also warned the eventual financial settlement could be higher, saying: "HM Treasury has based its estimate on a number of assumptions".

In a new report, the National Audit Office highlighted a series of ways the cost could be increased.

The UK could pay up to £3 billion more in budget contributions after its withdrawal than the Treasury estimates

This could happen if UK payments before we leave the EU in March 2019 are lower than expected.

There will also be a £2.9 billion payment to the European Development Fund.

This is a fund used by the EU to provide support to African, Caribbean and Pacific countries.

The UK has said it will honour its commitments to the fund, but this money wasn't included in the Treasury figures because the fund established under the EU treaties and sits outside the EU budget

There is an extra £7.2 billion cost to the Treasury

The Government's estimate is the net figure. That means it includes money that will be paid to the EU, but also takes into account money that the EU pays to the UK.

However, £7.2 billion in EU funding will go directly to the private sector. If we ignore this and just look at what the Treasury pays, it's £7.2 billion higher than the figure the Treasury has given.

There is more money the UK is liable for, but probably won't have to pay

The Treasury forecasts that the UK’s share of total outstanding contingent liabilities will be €14 billion as at the date of withdrawal. Additionally, the UK will provide a guarantee worth €35.7 billion to the European Investment Bank.

The chances of the UK ever having to pay this money are "remote" but not impossible.

Amyas Morse, the head of the National Audit Office, said: " We have reviewed the Treasury’s estimate of how much the UK will pay the EU under the draft withdrawal agreement.

"The estimate reflects a number of moving parts, so the range of costs in it could have been wider than £35 billion to £39 billion. But overall we think it is a reasonable estimate."

Theresa May faces a challenge from senior backbench MPs including Conservatives, who are to propose a motion to the House of Commons calling on the UK to stay in an "effective customs union" with the EU after Brexit.

The Government opposes remaining in a customs union, saying it would make it impossible for the UK to sign its own trade deals.

Labour has said it supports a continued customs union with the EU., although it wants to negotiate a new customs union rather than sticking to the existing arrangements.

Chairs of House of Commons Select Committees will present a motion to the Commons which "calls on the Government to include as an objective in negotiations on the future relationship between the United Kingdom and the European Union the establishment of an effective customs union between the two territories."

It will be proposed by MPs including former Conservative Education Secretary Nicky Morgan, Chair of the Commons Treasury Committee, Conservative backbencher Sarah Wollaston, chair of the Commons Health Committee, and Conservative former Attorney General Dominic Greive, chair of the Intelligent and Security Committee.

Labour MPs including Yvette Cooper, chair of the Home Affairs Committee, are also supporting the motion. Other Committee chairs backing it include Labour's Hilary Benn, Liberal Democrat Norman Lamb and SNP MP Pete Wishart.

In a joint statement, Ms Cooper and Ms Morgan said: "‘With just six months to go before the Brexit deal needs to be concluded, we are running out of time for Parliament to help to shape the negotiations.

"Yet many of our backbench committees have forensically gathered evidence on different Brexit options and the practical implications."

They added: "We both believe the case for a customs union is overwhelming - for the sake of British manufacturing, international trade, smooth borders and Northern Ireland peace.

"If Parliament stays silent until after the deal is done, that is just a recipe for conflict and regret later on. Far better for us to debate this properly now and let Parliamentarians express their view before it is too late."