With Brexit "done" as far as London is concerned, UK and EU negotiators on Monday begin talks aimed at forging a future relationship -- while brandishing red lines that may yet prove to be

irreconcilable.

The discussions covering trade and trading standards, agriculture, security, transport, energy, fisheries and police cooperation kick off in Brussels under teams run by Frenchman Michel Barnier for the EU and governmental advisor David Frost for Britain.

They take place just over a month since Britain ceased to be an EU member, and are meant to wrap up by the end of this year -- an exceedingly tight timeframe that few see as feasible for anything more than the barest of bare-bones accords.

- Mistrust -

The negotiations will be clouded by mistrust and weeks of chest-beating leading up to them. Each side has accused the other of shifting away from goals set out in a nonbinding political declaration struck late last year.

Rival mandates published last week highlighted the EU's aim of securing a "level playing field" to prevent Britain undercutting costly European standards on labour, tax, environment and state subsidies.

Meanwhile the UK is insisting on setting its own rules in the name of "economic and political independence".

Barnier, in an uncharacteristic display of podium-thumping, has told Britain to fully respect the binding Brexit withdrawal treaty.

That, notably, requires checks on British goods crossing the Irish Sea into the UK territory of Northern Ireland, which remains in the orbit of the EU single market.

Johnson has insisted those checks are unnecessary. But senior British ministers and officials confirm London will abide by the terms of the treaty.

- 'Posturing' ahead of the talks -

Experts say they see a potential landing zone for an agreement on future relations based mainly on trade in goods, but only if both sides give up a little ground from their political rhetoric.

"Clearly, at the beginning of any negotiation, there's a bit of posturing. Both sides want to state the strongest possible case," said Fabian Zuleeg, head of the European Policy Centre.

The EU demand for parallel standards and the UK's insistence on legal autonomy could be bridged in ways both sides claim as wins, they say.

But fishing -- of relatively minor economic importance but of totemic significance to Britain and EU states such as France and Spain -- could be the flashpoint that scuppers a deal.

Barnier has emphasised that fishing is "inextricably" linked to the whole agreement. The EU is demanding its fishing boats continue to have access to British waters in return for British fishermen being able to sell their fresh catches to their biggest and closest market.

That hump has to be resolved by the end of June when both sides are to get together for a stock-taking of the talks, to decide whether it is worth continuing.

- 'No Plan B' -

If no deal is reached, economic pain will be felt on both sides -- but especially in Britain and in Ireland, the EU member most dependent on trade with the UK.

UN economists calculate the trade hit would see the UK annually lose export revenues of up to 29 billion euros ($32 billion) in that case. The EU buys nearly half of all British exports.

Zuleeg said he was "pessimistic" about the outcome, because the UK was so strongly emphasising its desire to diverge from EU rules.

"It's very difficult to see where they could meet to make this work," he said. If the British government "stick to their line there cannot be a deal".

An EU official, speaking to AFP on condition anonymity, warned however that Brussels did not have an alternative negotiating line up its sleeve.

"There is only a Plan A, no Plan B. Because Plan B is to extend the negotiations and we know the British don't want that," the official said.

Another analyst, Jill Rutter, a senior fellow at think-tank The UK in a Changing Europe, said the fact that the EU is stressing there will be trade friction even if a deal is done, in the form of checks on goods entering the European single market, means "the benefits aren't enormous" for Britain.

Also, Johnson and his government appear ready for British businesses to bear the pain from a no-deal.

"What's very intriguing about this government, and what makes it so hard to judge, is that Johnson is apparently unwilling to take any account of business concerns," Rutter told AFP.

The main argument she sees in favour of a deal is that when Johnson decides he really wants to achieve a goal, he goes for it no matter his past positions.

"He's prepared to ditch prior commitments, and he's actually a very good sales person," she said.afp