London (AFP) - Donald Trump's election was expected to hand Britain a secret weapon in forging a post-Brexit future, but his interventions in British politics and controversial foreign policy have strained the so-called "special relationship".

The bond between Britain and the United States has been the backbone of the post-war geopolitical order, but after huge political upheaval in both countries, a status update on the special relationship might now read: "It's complicated".

Trump rode to the White House on a populist wave also seen in the Brexit vote. He appeared keen to help the UK by promising a swift trade deal once it left the European Union, in stark contrast to predecessor Barack Obama who had warned Britain would be "at the back of the queue".

Brexit supporters hailed Trump as "the tooth fairy", giving Britain an unexpected card in their negotiations with the EU.

But the US president's split with the international community over the Iran nuclear deal, his war-of-words with North Korea and his withdrawal from the Paris climate accord have strained relations and left Britain in a diplomatic fix.

"The idea that Donald Trump is going to come in on a white horse and fix all of Britain's trade problems is a myth," London School of Economics fellow Brian Klaas told AFP.

"It's a misplaced optimism because trade negotiations will take years to complete and Trump says things all the time that he doesn't mean and will never follow through on.

"Trump might not win in 2020, he might not be the person that the UK is dealing with," he added.

- State visit controversy -

Relations took a severe blow in September when the US imposed stiff tariffs on Canadian aerospace firm Bombardier, which employs thousands of people in Northern Ireland, following a complaint from Boeing.

Former defence secretary Michael Fallon warned the move "could indeed jeopardise our future relationship with Boeing," dashing hopes that the US would offer favourable terms in a future deal, which can only be officially negotiated after Britain leaves the EU in 2019.

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"There is still the possibility of a trade deal, but at the moment we can't foresee what that will look like," Russell Foster, foreign policy expert at King's College London, told AFP.

"The only indication we have got is what's happened in Northern Ireland with Boeing and Bombardier, where yet again Trump's unpredictable nature means 4,000 jobs are going to be lost."

British Prime Minister Theresa May was the first world leader to visit Trump's White House, and appeared to develop a bond, inviting the US leader for a state visit to stay with the Queen.

But hours after the meeting, Trump unexpectedly announced a travel ban from seven Muslim-majority countries, affecting dual British citizens and wrong-footing British officials, according to emails released by the BBC last week.

"The dual nationals angle will of course be really difficult for us," read one Foreign Office email.

"Not great after the PM visit," said another.

The offer of a state visit has reportedly been downgraded following a backlash to Trump's executive order.

There were threats of boycotts and mass protests, while speaker John Bercow declared the US president would not be allowed to address the House of Commons.

The row also strained personal relations between the two leaders.

"It's in Britain and America's interest to have close relations, but the political peril of that is huge," said Klaas.

- 'One-sided' relationship -

Trump has also infuriated British authorities with his tweets on terrorism in Britain, including highly publicised run-ins with London mayor Sadiq Khan.

However, while commentators and politicians rushed to condemn Trump, some of the criticism may be the result of the US leader's habit of highlighting volatile wedge issues that pit the establishment against sizeable chunks of the general public.

"As much as Trump is imagined by much of the elite as a bit of a clown, he's a very powerful symbolic figurehead for people who are dissatisfied and feel abandoned by establishment elites," said Foster.

The British establishment therefore finds itself in a bind, he added, with little choice but to persevere with a leader they see as a liability and who represents a movement that "absolutely" poses a threat to their very existence.

"The US provides us with economic and military benefits which far outweigh the benefits that we give to the Americans," Foster said.

"The special relationship has always been very one-sided."

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