Weary Brussels braced for a second bout with Boris

But there are some, among those who have known "Boris" down through the decades, who think he might be slippery enough to bluster his way through a successful divorce deal.

"There are two schools of thought," says Pierre Sellal, a former French ambassador to the European Union who knew Johnson as the Daily Telegraph's Brussels correspondent in the late 1980s and early 1990s.

"There are those who think the office makes the man, and that if he's PM he'll behave differently, and those who think that if the Foreign Office didn't change him, nothing will," he told AFP.

"I think we'll know which it is soon enough."

If, as seems likely, Johnson is chosen by Conservative MPs and party members to succeed Theresa May in Number 10 he has warned he will demand significant changes to Britain's Brexit withdrawal agreement.

If, as also seems likely, European leaders refuse to reopen talks on the deal or to tinker with the "Irish backstop" clause, he says he is ready for a "no-deal" Brexit.

This, economists and business leaders warn, would plunge trade relations between the UK and the remaining 27 EU members into chaos and perhaps tip Britain into recession.

So is Boris bluffing? And would he execute a dramatic U-turn if an overly hasty Brexit threatened an early and ignominious end to his premiership?

Popular tennis court

The former head of the World Trade Organisation and chief of staff to 1990s EU leader Jacques Delors, Pascal Lamy, has known the Johnson family since young Boris' Brussels childhood.

He recalls playing tennis on the Johnson family court in leafy Uccle, and later meeting Boris Johnson when he appeared as a journalist at European Commission press briefings.

He followed the ambitious young man's advance through journalism and politics, escaping scrapes that would have sunk others, to become mayor of London and then foreign minister.

For Lamy, Johnson's decision to become the figurehead of the Brexit referendum campaign, reflects his personal ambition rather than any deeply felt objection to EU integration.

"I don't think he has a very settled opinion on Brexit, but he has very settled opinion on himself. The only thing in which Boris Johnson believes is Boris Johnson," Lamy told AFP.

"Once he gets there he'll want to stay there. I think Brexit will be secondary to a strategy to remain prime minister."

Johnson may soften his stance now his main diehard Brexiteer rival Dominic Raab is out of the running, but he has long insisted Britain must leave the EU on October 31.

He says he would prefer a renegotiated deal but, if Europe does not bow to his demands in a short timeframe, he is ready to walk away when the deadline expires.

Brussels officials scoff at this.

After all, they argue, the damage to the British economy in abandoning tariff-free trade with no transition or standstill period would dwarf that to the much larger continental market.

But views differs as to what Johnson's arrival in Number 10 means for this equation.

Brexit campaign 'liars'

Some, like the trade expert Lamy, think Johnson will baulk and seek a way to kick the can down the road -- perhaps even warming to the idea of another referendum to resolve the matter.

"I can well see a scenario in which he's elected prime minister, comes to Brussels and makes his big scene," he said.

"Then, once they've told him 'No thanks, we've already tried everything and what's on the table is all there is', he heads home and says: 'Right then, friends, we need to decide, and it's you who'll do the deciding'."

But other insiders fear a different outcome.

French President Emmanuel Macron, the most outspoken EU leader in calling those who led the Brexit campaign "liars", has said he will oppose another extension to divorce proceedings.

Most in Brussels expect the French to fall into line -- now that European elections are over -- and agree once again to postpone Brexit, but some think Johnson's arrogance could thwart that.

"If he immediately acts aggressively, as he has begun to do, demanding a renegotiation, it'll all go bad fairly quickly," Sellal warns.

"The result will hasten the reversal in the mood in Brussels, and point the way to the exit, without or without a deal. Macron was in the minority, but his view could become the majority."