Cabinet secretary Sir Mark Sedwill says emergency contingency plans are needed.

Boris Johnson has said he would refuse to pay the promised £39bn to the EU unless it offers better terms on Brexit.

Boris Johnson has vowed to withhold Britain’s £39bn Brexit “divorce” payment until the EU agrees better terms for the UK to leave.

Boris Johnson, considered the frontrunner to succeed British Prime Minister Theresa May, said on Saturday he would refuse to pay the country's Brexit bill until the EU agrees better withdrawal terms.

He said he would deliver Brexit by refusing to pay the £39bn exit payment agreed by Theresa May until Brussels agrees to a better deal.

PARIS (Reuters) - A source close to French President Emmanuel Macron said on Sunday that failing to pay a 39 billion pound ($50 billion) Brexit bill when Britain leaves the European Union would amount to a sovereign debt default.

France’s state secretary for European affairs has confirmed that the EU27 are not prepared to reopen the Brexit withdrawal agreement, and that without a “new political line” in the UK or a second referendum, Britain must expect to leave the bloc on 31 October.

Budget chief linked £39bn divorce bill to UK retaining beneficial relationship with bloc. / A UK government led by Boris Johnson could not be dragged through the courts if it refused to pay the country’s multibillion-pound Brexit bill, the EU’s German budget chief has admitted, but Britain’s economy, security and universities would be made to suffer.

Philip Hammond says UK should stand by its existing obligations to the EU.

Boris Johnson’s claims about the prospects of rewriting the Brexit deal have been compared by the European parliament’s Brexit coordinator to the “false promises, pseudo-patriotism and foreigner-bashing” he is said to have used to win the EU referendum.

European Parliament Brexit chief says likely next prime minister is 'a man who continues to dissemble, exaggerate, and disinform the public about Brexit'.

Jeremy Hunt has declared that he would refuse to pay some of the £39bn bill it would cost the UK to leave the European Union.

Ministers accused of 'gagging order' as health chiefs told all communications must be 'aligned' with No 10’s 'top lines'.

Leo Varadkar says UK and EU will still have to negotiate, with same problems on agenda.

Iain Duncan Smith says the financial liabilities signed up to by Boris Johnson in the withdrawal agreement were too great.

Figure of £40.8bn buried in EU’s 2020 accounts dismissed by UK as not reflecting amount it will pay.

The Brexit "divorce bill" negotiated by Boris Johnson is up to £5 billion higher than the UK government expected, according to "definitive" EU calculations.

The UK is liable to pay €47.5 billion euros ($A75.7 billion) to the European Union as part of its post-Brexit financial settlement, according to the EU’s consolidated budget report for 2020.

The European Union has set the final Brexit divorce bill at £40.8 billion – well above where the UK Government expected it to be.

The Office for Budget Responsibility (OBR) predicted in 2018 that the so-called "divorce bill" for leaving the European Union would cost the UK between £35bn and £39bn - but the EU say the figure is over £40bn.