The United Kingdom will not be threatened into following European Union rules in the future by talk of economic rifts and is ready to trade with the bloc on basic international terms if needs be, Prime Minister Boris Johnson's Europe adviser said on Monday.

The UK left the EU last month and the two sides will now start negotiating a new relationship from trade to security.

"We are not frightened by suggestions there will be trade frictions," David Frost, Johnson's Europe adviser, said during a lecture at a Brussels university. "We are not asking for anything special, we are asking for a simple free trade agreement."

Should that not be possible because of the bloc's insistence that the UK aligns itself with EU rules ahead, London was ready to trade with the 27-country union according to the same basic international rules that Europe now follows with Australia.

The speech came as a response to one delivered recently in London by European Commission President Ursula von der Leyen, who called on Johnson to agree to so-called level playing field guarantees of fair competition based on ambitious environmental and labour standards.

For access to its market in the future, the EU demands these level playing field clauses to protect itself from any dumping from the UK.

"It is central to our vision that we must have the ability to set laws that suit us," Frost said. "To think that we might accept EU supervision on so-called level-playing field issues simply fails to see the point of what we are doing."

Frost, who described himself in the lecture as "one of the few Brexit-voting diplomats" in the country, said the UK would not extend the current, status-quo Brexit transition period that runs until the end of this year.

Frost, who said the UK would set out its own state aid system after the transition ends, described how he started his early career in Brussels as a Euro-enthusiast but soon became disillusioned.

He said that any easing of the Johnson government's Brexit stance would not be supported by the general public at home and that the EU must treat the UK as an equal if it wants to have a "durable and sustainable" new partnership.

He said it was "perfectly possible" for the EU and the UK to be economic competitors but political allies.