Democratic Unionist Party leader Arlene Foster | Charles McQuillan/Getty Images Northern Irish leader says Boris Johnson broke his word on Brexit The DUP has refused to support the Brexit deal Boris Johnson struck with the EU.

LONDON — Democratic Unionist Party leader Arlene Foster accused Boris Johnson of breaking his promise not to impose a border down the Irish Sea as part of his Brexit deal.

The Northern Ireland first Minister, whose party propped the Conservatives up in government under an informal deal after the 2017 general election, said: “Once bitten, twice shy.”

Leaked government reports have said goods checks will be needed in both directions between Northern Ireland and Great Britain under the deal Johnson struck with the EU.

The DUP has refused to support the agreement, after being promised by Johnson that he would not negotiate a trade border between parts of the U.K.

Asked on the BBC Radio 4 Today programme whether Johnson had “betrayed” her voters in Northern Ireland, she said: “I think it says more about the person who broke their word, than me and leadership of the unionist party, which is the largest party, the Democratic Unionist Party.”

A Treasury report leaked by Labour on Friday said that under the government’s Brexit deal, businesses would have to fill in forms to export goods from Northern Ireland to Great Britain, and suggested that such checks may also be required in the other direction. The document warned that this would be “highly disruptive” to the Northern Irish economy and sets a precedent for “differential treatment for a constituent part of the U.K.”

But Johnson insisted on Sunday during an interview with Sky's Sophy Ridge, “there won’t be checks.”

“We’re part of the same customs territory and it’s very clear that there shall be unfettered access between Northern Ireland and the rest of Great Britain,” he said, adding the Treasury report was "wrong."

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