British Prime Minister Theresa May says US President Donald Trump had previously advised her to sue the European Union as part of her Brexit strategy, disclosing a piece of advice Trump says last week she had ignored.

Theresa May faces her first nationwide electoral test as Britain's prime minister, as she toughens her Brexit position just weeks before a crucial general election.

Britain's Theresa May calls for snap elections on June 8, catching the country by surprise in a bid to bolster her position before tough talks on leaving the European Union.