Arlene Foster, the leader of the Democratic Unionist Party, walks through the venue of the Conservative Party Conference in Birmingham, Britain, October 2, 2018. REUTERS/Toby Melville

LONDON (Reuters) - The Northern Irish party which props up Prime Minister Theresa May warned that its “blood red” line was that there could be no Brexit deal that would divide Northern Ireland from the rest of the United Kingdom.

The border between Northern Ireland and the Republic of Ireland is the last major sticking point in Brexit talks, though both sides are trying to work out how to monitor and regulate trade over the border.

“There cannot be a border down the Irish sea, a differential between Northern Ireland and the rest of the UK,” DUP leader Arlene Foster told the BBC on Tuesday. The interview was aired on Wednesday. “The red line is blood red.”

“All along we have said: ‘No new regulatory alignment’,” Foster said.

When asked if she was prepared to vote down PM May on a Brexit deal, she said: “We don’t want to be in that position.”

“This is too important to be playing around with things because this is the union - this is what brought me into politics,” Foster said.

The DUP’s 10 lawmakers prop up May’s minority Conservative government.