Conservative MP Boris Johnson arrives at the Conservative Party Conference at the ICC, in Birmingham, England, Tuesday Oct. 2, 2018. (AP Photo/Rui Vieira)

Birmingham - Britain's Boris Johnson on Tuesday launched a blistering attack on Prime Minister Theresa May's Brexit plan at a fractious party conference that came as Brussels pressed London for a compromise on leaving the EU.

Mixing breezy jokes with biting one-liners that always stopped short of attacking May herself, the former foreign minister called her strategy "dangerous and unstable".

"My fellow Conservatives, this is not democracy. This is not what we voted for. This is an outrage," he said in a 35-minute address that was interrupted by repeated bursts of laughter and cheers.

The wide-ranging speech was widely seen as Johnson's audition for May's job before the party faithful at their annual gathering.

Some 1,500 people packed a conference hall room after queueing for hours to see May's political nemesis deliver his biggest address since he quit the government in protest in July.

Johnson did little to douse suspicions of an eventual leadership challenge, using his characteristic swagger and wit to pick apart May's proposal -- while not actually spelling out what Britain should do instead.

He said he wanted to force May to reverse her bid to keep close economic ties with the European Union once Britain leaves in March. 

May appeared intent on stealing Johnson's spotlight, scheduling a flurry of morning interviews at which she unveiled a new immigration policy a day ahead of schedule.

Free movement of EU workers was a key issue in the 2016 vote for Brexit, and May confirmed that Europeans would be treated the same as non-EU citizens in future.

She took to the airwaves again after Johnson's appearance, telling Sky News dismissively that he "always puts on a good show" while she focuses on things that mattered to people's "day-to-day lives".

May's public battle with Johnson comes only two weeks before she must return to Brussels in a bid to break the Brexit deadlock and reach a deal.

The prime minister hinted at a possible compromise on how to keep the land border with Ireland open after Brexit, a key sticking point in the EU talks, on which London is due to publish new plans shortly.

- Alternative plan -

Johnson was a leading campaigner for Brexit in the referendum. Even after being appointed foreign minister, he continually challenged May's EU strategy.

He quit after May got the remaining ministers to rally around her proposal for Britain to follow EU rules for goods after Brexit.

May says the so-called "Chequers" plan will keep trading lines open with its biggest trading partner.

Johnson argues it will keep London tied to the bloc and ignore the people's expressed wish to leave the bloc.

The EU has also rejected May's vision, saying it undermines the its cherished single market.

Numerous eurosceptic Conservative MPs have already drawn big crowds in Birmingham as they offer their own idea for a looser trade arrangement with the EU.

Johnson endorses the same alternative proposal, but the mop-haired former London mayor has particular star appeal.

He was mobbed by jostling cameras on arrival and chased by reporters as he walked, with his address airing live across the nation.

"It's important to support the only politician I can see who is actively campaigning to give people the Leave that they voted for," said Colette Wyatt-Lowe, 71, a councillor from the outskirts of London, while queueing to hear Johnson speak.

- 'He's a clown' -

Johnson is known for his confrontational style and before the event he was captured in a press photograph running through a field of long grass.

It clearly mocked May, who once said the naughtiest thing she had done was run through a field of wheat.

But Johnson's recent comments about her Brexit plan, calling it "deranged" and saying it strapped a suicide vest to the British constitution, have angered many fellow MPs.

Several former colleagues used the conference to make jokes at his expense, while finance minister Philip Hammond said his alternative Brexit plan was "fantasy".

Even former Brexit minister David Davis, a fellow eurosceptic who describes Johnson as a "great mate", said many of his ideas "are good headlines but not necessarily good policies".

Among the delegates, there is also unease about Johnson's public disagreements with May.

Mike Kay, a 34-year-old delegate from Manchester in northwest England, summarised the mood of Johnson's critics.

"A large section of the country thinks he is a clown," he said.

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