Christopher Furlong | Getty Images | Getty Images News

Making matters worse, the shock result plunged one of America's closest allies and a critical trading partner into uncertainty just 10 days before the U.K. enters very complex and high-stakes talks over its "Brexit" from the European Union. The Conservatives remain Britain's largest party, but May's failure to secure an overall majority could put her alliance with Trump to the test. It also seriously depletes her own political power. "The result is a major embarrassment for May, who called the election just a few weeks ago on the back of a commanding 20-plus poll lead," Kallum Pickering, senior U.K. economist at investment bank Berenberg, said in an email to CNBC.

Tough times for the 'special relationship'

May has voiced concerns about some of Trump's hard-line views — and Trump is intensely disliked in the United Kingdom — but the Tory leader recently reaffirmed her commitment to maintaining the U.K.'s "special relationship" with the United States. May's political standing and Britain's economic health are especially important for the United States now, when so much of the rest of America's alliance with Europe is on the ropes. Trump has openly attacked Germany — he famously said that country is "very bad" for the United States — and the U.S.-Germany relationship is in its worst state since Germany reunified in 1990. Trump has spoken with open contempt for the European Union and the NATO military alliance, and he has professed admiration for Russian President Vladimir Putin at a time when much of the rest of the Western world sees him as tremendous threat. May's willingness to set aside political differences with Trump was perhaps most fittingly captured when she was photographed holding hands with the president in her first state visit to Washington.

British Foreign Secretary Boris Johnson speaks ahead of Prime Minister Theresa May during her last campaign visit at the National Conference Centre on June 7, 2017 in Solihull, United Kingdom. Carl Court | Getty Images