LONDON (AP) — A respected British think tank has slammed Prime Minister Boris Johnson's Brexit deal, concluding that the economy would be 3.5% smaller compared with staying in the European Union.

The National Institute of Economic and Social Research study released Wednesday says the agreement would deliver a 70 billion-pound ($90 billion) blow to the UK.

The researchers based their prediction on the assumption that the U.K. would leave the bloc with a free trade agreement with the EU after a transition lasting until 2021 while negotiating new deals with other nations.

Britain's government says it plans a "more ambitious" trade deal with the EU than the one considered by the think tank.

The research suggested a no-deal Brexit would cause an even greater loss to the economy, with a 5.6% blow to GDP.