Exclusive: Sir Peter Westmacott, Britain's ambassador to the United States until 2016, tells Business Insider that it will be "extremely difficult" for the UK to negotiate a free-trade deal with the United States.

Brexit-supporting MPs have consistently trumpeted the prospect of a UK-US trade deal as a means of compensating for the economic hit from leaving the EU.

"I think this is all going to be extraordinarily difficult and will take a long time to negotiate," said Westmacott.

LONDON — Britain's hopes of a quick post-Brexit trade deal with the Trump administration are "not going to happen" the UK's former ambassador to the US has told Business Insider.

Sir Peter Westmacott, who served in Washington during the Obama era between 2012 and 2016, told Business Insider that Downing Street's suggestion that Britain will be able to forge a speedy, generous trade deal after Brexit would be "extraordinarily difficult" to fulfill.

"Number 10 is still hanging onto Donald Trump's statement of support for Brexit, and promises that there will be a nicely-crafted free-trade agreement ready to go as soon as Brexit takes effect," said Westmacott.

"That's not going to happen. It is going to be extremely difficult for the UK to negotiate a free trade deal with the USA."

"[It's] not going to happen. It is going to be extremely difficult for the UK to negotiate a free trade deal with the USA."

Westmacott said that while a quick deal was technically possible, in practice Britain's negotiating clout would be significantly smaller outside the EU, which would tilt the balance of talks heavily in Washington's favour.

"I firmly believe [...] that our negotiating clout is much greater if we are in a group of 28 countries with a population of 450 million mostly prosperous consumers negotiating with America, with a population of 350 million people — a broadly similar sized economy and population — rather than negotiating on our own as a much smaller economy," he said.

"If the size of your market is much smaller, there's always going to be an imbalance in the negotiations. I think a deal of sorts would be attainable. But would it be a good deal — a genuine, comprehensive deal? I think not. If you wanted a quick deal it would probably be an inadequate one."

Brexit-supporting MPs have consistently trumpeted the prospect of a transatlantic trade deal as a means of compensating for the economic hit from leaving the EU. Foreign Secretary Boris Johnson boasted last year that Britain would be "first in line" to do a trade deal with the US, while International Trade Secretary Liam Fox boasted of dozens of new trade deals being ready to sign "the second after" Brexit.

But recent signals have been less promising. When they flew to Washington for trade talks in July last year, UK negotiators were alarmed to discover their US counterparts offering significantly worse terms than the UK currently enjoys on key treaties such as the 'Open Skies' agreement, which dictates the terms on which planes fly across the Atlantic.

'America First'

President Donald Trump and Britain's Prime Minister Theresa May react during a ceremony at the new NATO headquarters before the start of a summit in Brussels, Belgium, May 25, 2017. REUTERS/Christian Hartmann Perhaps more alarmingly, Trump's nascent war on global free trade is set to hit Europe within weeks, with the European Union bracing for a trade war after Washington indicated it would not extend the exemption it had granted to European importers from highly punitive tariffs.

Westmacott said that a free-trade deal with the United States would always have been difficult to arrange, because of Britain's comparatively small market and because the UK's desire to access lucrative procurement contracts would likely be blocked by protectionist state laws.

But he said the Trump administration's hostility towards free trade made a favourable deal within the next few years considerably more unlikely.

"While it would always be a tough deal [...] with a Trump administration that believes in "America First," which in "fair trade" not free trade, it would probably be even more difficult to get a decent free trade agreement than it would have been with a more pro-free trade US government," he said.

"I think this is all going to be extraordinarily difficult and will take a long time to negotiate," he added.