Britain is the United States’ closest ally but their long friendship may be sorely tested as the two countries try to forge a new trade agreement after Britain’s exit from the European Union.

Axar.az reports citing foreign media, U.S. Treasury Secretary Steven Mnuchin said on Saturday in London that he was optimistic that a bilateral deal with Britain could be reached as soon as this year.

But Mnuchin gave up no ground after a second meeting with his UK counterpart, Sajid Javid. Javid has insisted that Britain will proceed with a unilateral digital services tax, despite a U.S. threat to levy retaliatory tariffs on British-made autos.

Mnuchin told reporters after Saturday’s meeting that such taxes would discriminate against big U.S. tech companies like Alphabet Inc’s (GOOG.O) Google, Apple Inc (AAPL.O), Facebook Inc (FB.O) and Amazon.com (AMZN.O).

The UK Treasury declined to comment on the private meeting.

The divide highlights the challenges ahead as the Trump administration seeks a new bilateral agreement with Britain, part of a broader push to rebalance relations with nearly all its major trading partners.

The stakes are high - British Prime Minister Boris Johnson has pegged the trade deal with United States as a way to ease the pain of breaking with Europe, Britain’s largest trade partner. U.S. President Donald Trump, has promised a “massive” trade deal to support Brexit, the product of a populist movement similar to his “America First” agenda.