Donald Trump's state visit may do Britain a favor - just not in the way he envisages. Even before Air Force One touched down in London last week, the US president had intervened in the race to succeed Prime Minister Theresa May, advocated a chaotic Brexit, and insulted the capital's mayor, Sadiq Khan. Leaving the European Union makes Britain even more dependent on an increasingly erratic ally.

Trump's endorsements will probably backfire. President Barack Obama's intervention in the Brexit debate in 2016 did not stop Britons from voting narrowly to leave the EU. The public will be even less receptive to unsolicited advice from Trump, who is much more unpopular in the UK than his predecessor.

So when Trump says Boris Johnson would be a good choice to replace May as Conservative Party leader, or recommends that arch-Brexiteer Nigel Farage be involved in negotiations with the European Union - both of which he did in recent newspaper interviews - he actually makes these things less likely to happen.

The president's antics also undercut the enthusiasm of pro-Brexit politicians for a free trade deal with the US, which they argue would help make up for leaving the EU's single market. Demands that Britain open up its markets to American chicken and pharmaceuticals already face resistance. 

But Trump's recent decision to impose tariffs on Mexico - just six months after signing a new free trade deal with the country - underscores his lack of reliability. Many pro-Brexit politicians will support the president's quest for reduced immigration and a more muscular approach to trade. But Trump's willingness to wield these diplomatic tools is also a painful reminder of Britain's diminished global clout. American pressure on the UK government to ban telecom equipment from Huawei Technologies, the Chinese group, is just a taster of future demands.

Britain's relationship with the US will survive well after the current president has vacated the White House. Even so, during his tenure America has become a less reliable partner - a fact no number of royal photo opportunities, state banquets or wreath-laying ceremonies can paper over.

The author is Peter Thal Larsen, a Reuters Breakingviews columnist. The article was first published on Reuters Breakingviews. bizopinion@globaltimes.com.cn