London (CNN Business) A free trade deal with the United States could add just $4.3 billion to the UK economy over the next 15 years, the British government estimated Monday. The modest potential windfall underscores the difficulty of replacing the economic benefits of membership in the European Union.

On Monday, the government published its objectives for trade talks with the United States , with Prime Minister Boris Johnson promising that the "best negotiators in the business" will hammer out a deal that boosts exports of UK products ranging from dairy to cars.

A deal could increase the size of the UK economy by as much as £3.4 billion ($4.3 billion), or 0.16%, over the next 15 years, the government said. Transatlantic trade flows would increase by an estimated £15.3 billion ($19.5 billion) over the same time period.

"Striking ambitious free trade agreements with our partners around the world is one of the key opportunities of Britain becoming an independent trading nation once again," Liz Truss, the UK International Trade Secretary, said in a statement. The government said it hoped that new agreements would cover 80% of Britain's trade by 2022.

Reaching a deal with America could prove difficult, however, because Washington lacks a big incentive: US companies already export more to the United Kingdom than the other way round.

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