A forecast prepared for the British government reportedly says the economy will be worse off after the country leaves the European Union whatever trade deal is struck with the bloc.

The leaked assessment published Monday by news website BuzzFeed says if Britain reaches a free trade deal with the EU, growth will be 5 percent lower than current forecasts over 15 years.

The document, dated this month, says if there is no deal and Britain resorts to World Trade Organization terms, growth will fall 8 percent. If Britain remains in the EU single market but as a non-bloc member, the decline will be 2 percent.

The government maintains that Britain will be able to strike new trade deals around the world after Brexit to replace any decline with the EU.