One thing is certain about Brexit: The U.K. will have to make a big payment to the European Union on its way out of the 28-nation bloc.
It's just not clear how much -- estimates range widely.
Boris Johnson, the U.K. foreign secretary, said Tuesday that he believes the figures being discussed are "extortionate."
The money would honor the multi-year spending commitments that the U.K. previously made alongside other EU members.
"The sums that I have seen that they propose ... seem to me to be extortionate, and I think 'go whistle' is an entirely appropriate expression," Johnson said to members of the U.K. House of Commons.
By the term "go whistle," he meant -- the EU can ask for it but it won't get it.
EU Commission President Jean-Claude Juncker said in March that Britain will need to pay roughly $64 billion as it leaves the EU. Other estimates have pegged the figure between $29 billion and $114 billion.
Related: The CNNMoney Brexit Jobs Tracker
EU member states pay into a communal budget, which finances infrastructure projects, social programs, scientific research, farm subsidies and pensions for EU bureaucrats.
The bloc's budget is negotiated to cover a period of years, with the current agreement extending to 2020.
Brexit is slated to happen in early 2019.
British Prime Minister Theresa May has previously threatened to walk away from Brexit negotiations without paying.
Johnson, who previously served as mayor of London, was a leading campaigner for the U.K. to leave the European Union.
He's a colorful character known for his quips and gaffes and his rivalry with former prime minister David Cameron.
In 2013 he gained added notoriety for calling a group of London's elected officials "great supine protoplasmic invertebrate jellies."
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Johnson may talk tough, but the U.K. is under huge pressure to negotiate and pay a Brexit bill that will satisfy EU leaders.
Otherwise the country risks alienating its biggest trading partner: EU states provide a market for 44% of all British exports and supplies 53% of its imports.
