EU chief negotiator Michel Barnier underlined Wednesday there will be no post-Brexit trade deal without agreement on fisheries, warning Britain against using the issue as a "bargaining chip" in stalled talks.

"There will be no trade agreement between the UK and EU... without a sustainable agreement and balanced agreement on fisheries," he told Irish think-tank the Institute of International and European Affairs.

"The UK has not shown any willingness to seek compromises," he added, speaking via video-link.

Barnier said London's position, which calls for annual negotiations on fishing quotas and access, could "lock out" European fishermen from UK waters.

He noted they had been fishing there "long before" Britain joined the European Economic Community -- the precursor to the European Union -- in 1973. 

"We will not accept that the work and the livelihoods of these men and women be used as a bargaining chip in these negotiations," Barnier said.

Britain formally left the EU in January, nearly four years after a landmark referendum to end almost 50 years of European integration.

But it is currently in a standstill transition period until the end of 2020 as the government tries to negotiate a new trade deal with the bloc.

If no accord is finalised by December 31, EU fishing fleets fear being locked out of rich waters surrounding Britain.

Prime Minister Boris Johnson's spokesman said that while a fisheries agreement is possible "it's clear that that's not going to be easy to achieve".

He told reporters in London the EU is insisting on sealing the fisheries deal before progressing other strands in the talks, "making it very difficult to progress".

Barnier said the trade deal must be concluded by the end of October to allow the European Parliament and European Council member states "to have their say".

He added "important questions remain open" on Britain's implementation of the protocol for Northern Ireland, around goods arriving and leaving and checks on them.

The province, which saw three decades of sectarian violence between nationalists who favour being part of Ireland and Protestant unionists who want to remain in the UK, will be the nation's only land border with the EU post-Brexit.

"The UK still needs to complete many, many practical preparations -- time consuming and resource intensive preparations," Barnier said.