The assurance came after May held a series of meetings with Brexit supporters in her Cabinet amid reports that David Davis, the minister leading negotiations with the European Union, had threatened to resign because an earlier proposal could have forced Britain to follow the bloc's trade rules indefinitely.
At the same time the United Kingdom would be able to strike free trade agreements with other countries and to implement those elements which did not affect the functioning of the backstop.
But the customs boss defended the figure when he appeared before MPs and said a "similar" amount would be lost if the government fails to secure an agreement with Brussels.
The government has proposed amendments to its Brexit legislation after the unelected upper house of parliament made 15 changes to British Prime Minister Theresa May's original plans, despite ministers' attempts to block them.
In its manifesto Best for Britain argues that both sides of the Brexit debate should welcome a second poll in less than three years to finally settle the question of the UK's membership of the European Union and that the issue was diverting time and energy from other political concerns.
Theresa May has ruled out the plan as it would effectively crate a barrier in the Irish Sea between Northern Ireland and the rest of the UK.
DUP deputy leader Nigel Dodds said: "The backstop in the technical report issued applies to the entire United Kingdom".
He added: 'The UK has chose to leave the European Union we respect that democratic decision and are implementing it.
Labour MP Chris Leslie, who is campaigning for a "People's Vote" on the final Brexit deal and believes the United Kingdom should stay in the single market and customs union, said the row "would be laughable if it weren't so serious".
'I welcome the publication of the document, bu being very frank I think it raises more questions than it provides answer'. "The temporary backstop is not in line with what we want or what Ireland and Northern Ireland want and need".
'The Commission's proposals did not achieve this, which is why we have put forward our own backstop solutions for customs. "This is the trucking equivalent of the threat to the aviation sector because of the ending of Europe-wide agreements when the United Kingdom leaves the Single Market".
'Michel Barnier has confirmed today that discussions will now continue on our proposal'.
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