Michel Barnier mocks foreign secretary Boris Johnson's refusal to consider large Brexit bill.

Chief EU negotiator warns that "clock is ticking" for Britain to settle its accounts.

Barnier denies that the EU is seeking "revenge" against the UK for Brexit.

LONDON — The EU's chief negotiator has mocked the foreign secretary's claims that the EU can "go whistle" if it expects a large Brexit divorce bill from the EU.

The EU is reportedly expected to demand up to €60 billion from the UK as part of Brexit negotiations.

Boris Johnson told the House of Commons on Tuesday that the EU were demanding "extortionate" sums from Britain.

"The sums they [the EU] propose seem to be extortionate and I think 'go whistle' is an entirely appropriate expression," he said.

Asked about Johnson's comments, the EU's chief negotiator Michel Barnier warned that time was running out for the UK to get a Brexit deal.

"I am not hearing any whistling, just the clock ticking."

Barnier said the UK must meet its financial obligations or risk breaching trust for its future relationship with the EU.

"How can you build a future relationship with a country where you don't have trust?" he said.

"I cannot imagine this great country - which the UK is - not also being a responsible country, and respecting its commitments."

Barnier said the EU would refuse to progress negotiations onto Britain's future trading relationship with Europe until it agreed to settle the divorce bill.

He added: "The United Kingdom must recognise its financial obligations..."

He told reporters that: "Trust means [the UK] settling its accounts..."

"It is not a ransom, it is not an exit bill, it is not revenge. It is just settling accounts."