BRUSSELS • British and EU trade negotiators have embarked on high-stakes talks aimed at forging a new post-Brexit relationship, with the timing tight and both sides far apart on key issues.

The EU's chief negotiator Michel Barnier and his UK counterpart David Frost met in Brussels on Monday, launching several months of intense talks involving around 100 officials on each side.

The negotiations began just over a month after Britain left the European Union, and are meant to wrap up by the end of this year - an exceedingly tight timeframe that few see as feasible for anything but a bare-bones accord.

The deadline is Dec 31, the end of the UK's current transition period during which it trades like an EU member with no tariffs or other barriers.

British Prime Minister Boris Johnson has ruled out extending the transition and both sides are looking to an EU-UK summit in June to decide whether talks are worth continuing.

The negotiations have been clouded by mistrust and weeks of chest-beating as each side has accused the other of reneging on high-ambition goals set out in a political declaration struck last year.

This week's talks in Brussels will end tomorrow, after two days in which the teams will divide to cover 11 key topics for the future relationship.

The next round will take place in London, and thereafter alternate between the two capitals.

Mandates published last week highlighted the EU's aim of securing a "level playing field" to prevent Britain from undercutting 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".

Experts warned that the two sides are on a collision course, with a deal highly unlikely without concessions.

"A deal between the EU and the UK by the end of the year is still possible, but it will require both parties to drift away from their opening positions," said Mr Sam Lowe, a senior research fellow at the Centre for European Reform.

"In practice, it requires the UK to move a lot, and the EU to move a little," he added.

Underlining the tensions, Mr Barnier, in an uncharacteristic display of podium-thumping, has warned Britain that any backsliding on its EU divorce terms would torpedo trade talks.

The Brexit deal notably requires checks on British goods crossing the Irish Sea into the UK territory of Northern Ireland that Mr Johnson now says are unnecessary.

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.

Mr Barnier has emphasised that fishing is "inextricably" linked to the whole agreement.

The EU is demanding that its fishing boats continue to have access to British waters in return for British fishermen being able to sell their catches to their biggest and closest market.

If there is no broad trade deal, 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.

AGENCE FRANCE-PRESSE