Prime Minister Boris Johnson on Wednesday said a post-Brexit trade deal was still possible, as he prepared to head to Brussels to meet European Commission chief Ursula von der Leyen.

"A good deal is still there to be done. I look forward to discussing it with Commissioner von der Leyen tonight," he told parliament.

But Johnson undermined hopes of a breakthrough by again taking aim at European efforts to ensure Britain still abides by competition rules after January 1.

The so-called level playing field has been a key stumbling block in the talks, as has the extent of EU access to British fishing waters.

Johnson said Brussels' proposals on fishing would mean Britain would be "the only country in the world not to have sovereign control over its fishing waters".

"These are not terms that any prime minister of this country should accept," he told lawmakers in his weekly question and answer session.

Britain formally left the EU on January 31, nearly four years after a referendum on membership that divided the country and paralysed politics.

The end of a transition period, designed to allow both sides to thrash out the terms of their new relationship, is approaching on December 31, making agreement on a deal more urgent.

Whatever happens, Britain will leave the European customs union and single market. Johnson said deal or no deal, the country will "prosper mightily".

By retaking control of UK money, borders and laws, "we will seize all of the opportunities that Brexit brings", he said.

But Johnson is under strong pressure from businesses, which want greater clarity on the new rules that will apply from January 1.