LONDON (AFP) - Anti-EU populist Nigel Farage urged Prime Minister Boris Johnson on Friday (Nov 1) to follow US President Donald Trump's advice and join forces with his Brexit Party in Britain's pre-Christmas general election.

Mr Trump used a lengthy appearance on Mr Farage's radio phone-in show Thursday to share his opinion on Britain's Dec 12 snap poll just as campaigning got under way.

Mr Trump urged Mr Johnson and Mr Farage to "get together" in the election campaign and forge "an unstoppable force", and warned that opposition Labour Party leader Jeremy Corbyn would be "so bad for your country".

He also cautioned that Mr Johnson's EU divorce deal meant that "under certain aspect... you can't trade" with the United States.

Mr Johnson has publicly resisted the idea of forming an official union with Mr Farage, whose Brexit Party is struggling in the polls.

The eurosceptic is considered a polarising figure who might push away the Conservatives' more moderate supporters.

Mr Trump's intervention became immediate top news in Britain and drew hasty replies from all sides.

Mr Farage made headlines just as his Brexit Party prepared to launch its campaign.

The veteran European Union critic went back on radio on Friday morning to tell Mr Johnson - a fellow campaigner for Brexit during Britain's 2016 EU membership referendum - to heed Mr Trump's advice.

"Without wanting to hurt Boris Johnson, he told the truth about this shocking, awful EU treaty," Mr Farage told LBC radio.

"It's doubtful even by 2023, we would even be in a position to have a trade deal with the USA," he added.

"If that applies to America, that applies to Japan, it applies to India, it applies to Australia. For goodness sake, Boris, drop the deal and let's start again with a clean sheet."

He promised to use his party's campaign launch to explain "the need, in my view, for some kind of Brexit alliance" with Johnson's Conservatives.

Mr Corbyn tweeted on Thursday that Trump was "trying to interfere in Britain's election to get his friend Boris Johnson elected".

The White House chief - due in Britain for a Nato leaders summit in early December - remains deeply unpopular in Britain and has exchanged barbs with Mr Corbyn in recent months.

But Mr Trump harbours his own suspicions of Brussels and has embraced Mr Johnson's efforts to get Britain out of the European Union after nearly a half-century of membership.

Reacting to Mr Trump's criticism of the EU deal, Downing Street said Mr Johnson's reworked withdrawal agreement allowed Britain to strike its "own free trade deals around the world from which the UK will benefit".

A UK poll of polls shows his Conservatives leading Mr Corbyn's Labour by 36 to 25 per cent margin.

But support for Mr Farage's party has slipped to 11 per cent. His party has struggled since winning the European Parliament elections Britain was forced to take part in May because of repeated Brexit delays.

Mr Johnson might need to secure a majority of the seats in the Lower House of Commons to stay in power and avoid the feat of becoming one of Britain's shortest-serving prime ministers.

His minority government's old alliance with Northern Ireland's Democratic Unionist Party (DUP) is in disarray because of disagreement over Mr Johnson's new Brexit deal.

The other smaller parties all back Europe and are more likely to try to form a coalition designed to stop Brexit.

"We are not negotiating with the Brexit Party," a Downing Street source told The Sun newspaper.

"We do not do electoral pacts."