Britain is one of the biggest contributors and its departure will leave a huge hole in the European Union's finances.
The rest of the EU wants a deal to ensure all the plans agreed during the UK's period as a member continue to be funded.
Lawyers for the UK government have reportedly advised we are not obliged to pay a penny when we leave the bloc.
But striking a trade deal is impossible without some kind of settlement and EU officials demanded up to £100billion.
Theresa May has said Britain will pay its dues, but the exact figure has been the subject of fraught negotiations.
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This graphic explains the amount initially demanded by the EU
How much will it cost taxpayers?
Theresa May has tabled a whopping offer to the EU in a bid to move talks forward and start negotiating a future trade deal.
The UK is set to pay between £35 and £39billion in a divorce bill, spread over the several decades - but Theresa May has said the offer will only stay on the table if a transition deal is reached by March.
The details of the deal are causing unrest among Brexiteers, even though at £35billion to £39billion the financial settlement is less than many had feared.
But the agreement, reached on December 8, negotiations can move onto talking about the vital transitional deal and a long-term agreement on future trade.
Theresa May said the settlement, which includes Britain’s share of the EU’s budget to the end of 2020, as well as outstanding debts and liabilities such as pensions of staff at European institutions, was "fair to the British tax payer".
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