Some hardline Brexiteers have blasted Prime Minister Theresa May for saying Britain will pay for its “liabilities” to settle up the account with the EU which could be in the region of £40bn.
Figures released by the Office for Budget Responsibility (OBR) predict the cost of the UK staying in the European Union for just four more years after the Brexit leaving date of March 2019 would be £55bn.
The numbers, released on Budget Day, show that UK payments to the EU would rise sharply over the next few years, if it remained a member.
Transfers are set to rise by 25 per cent next year from £9.9bn to £12.5bn.
This figure is due to rise to £13.8bn the year after.
Conservative MP and former party leader Iain Duncan Smith said the figures revealed leaving the EU will be “a bargain” even after settling the divorce bill.
Mrs May is expected to lodge a new offer – believed to be in the region of £40bn – with the EU on Monday in a bid to move the Brexit talks on to sorting out a trade deal.
The OBR was forced to defend itself against accusations it had made “anti-Brexit” predictions and exaggerated the costs of leaving the EU.
Chair of the watchdog Robert Chote told MPs on the Treasury select committee that accusations of bias were “without any foundation whatsoever”.
He said: “We’ve explained how the Brexit assumptions are done. We look at the range of outside analysis.
“As we highlight throughout, obviously it depends on what the deal is with the EU and it depends on what you do in other countries as well.”
He added because of the size and closeness of the EU to Britain any disruption in trade between the two are likely to have an impact on the UK economy during the OBR’s five-year forecast.
Mr Chote said that making any alternative assumptions in the forecasts, which was not backed by most economists, would produce justified accusations that the our was seeking to flatter government officials with an overly optimistic outlook.
The OBR recently downgraded its assessment of the British economy in the medium term from more than 2 per cent in the March Budget to 1.5 per cent in the November Budget.