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, AP reported.
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.
The leaked analysis says new trade deals with the U.S., China, India and other countries would boost growth, but not enough to make up for the lost revenue from the EU, currently Britain's biggest trading partner.
The government did not challenge the authenticity of the document, but said Monday it would not comment on its internal Brexit analysis.
The report is the latest in a string of economists' predictions that leaving the EU will harm the UK economy. Supporters of Brexit point out that many such forecasts have so far not come to pass.
The document is likely to further inflame feuding within Britain's Conservative government, between those who back a clean break with the bloc and those who want to keep Britain's economy as closely aligned with the EU's as possible after Brexit in March 2019.