Britain faces paying nearly an extra £850 million into European Union coffers if it remains part of the single market, campaigners have claimed.
Brussels has "secretly" raised spending limits by £1 billion on the long-term budget that had been curbed after demands by former prime minister David Cameron in 2013, according to Change Britain.
It also claimed European Commission documents suggest payment ceilings in the financial plan, which runs to 2020, will be breached in its final two years.
The group, which was set up in the wake of the referendum to campaign on the terms of Britain's departure, said the UK could be left with a total additional bill of £845 million.
Cameron, along with Germany, the Netherlands, Finland and Sweden, secured an agreement for real terms cuts in the budget, known in Brussels jargon as the multi-annual financial framework (MFF), amid continuing austerity following the financial crash.
Change Britain said the UK must quit the single market when it leaves the EU on the current 2019 timetable to avoid being landed with extra bills.
It claims that remaining part of the trading agreement would leave the country "almost certainly" having to pay into the budget in the same way it does now.
Conservative Dominic Raab, one of its founding supporter, said: "EU politicians promised David Cameron that the EU would cut its budget. These figures show that eurocrats are now going back on their word and will ask for even more money from UK taxpayers if we stay in the EU's single market.
"Millions of people voted Leave on June 23 because they think that the EU costs us too much cash. Instead of sending more money abroad people voted for that money to be spent here on our priorities like the NHS.
"People are fed up with being ripped off by Brussels. We now need to work to leave the EU as quickly as possible and take back control of our money."