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However, she is said to be ready to accept a number of regulatory checks between mainland Britain and Northern Ireland if Brussels moves to accept a UK-wide backstop.

The Prime Minister has stood firm in rejecting the EU’s backstop offer should would effectively annex Northern Ireland from Great Britain by leaving the country attached to the bloc’s custom regime.

Mrs May has struggled to offer concessions on the thorny Irish border issue because her minority Conservative Government requires the support of 10 DUP MPs.

Ms Foster, the Prime Minister's kingmaker, met Michel Barnier in order to reinforce her commitment to her party's "blood red lines" she has set out on th Irish border.

As we leave the EU, we should do so as one nation

Ms Foster is wholly opposed to checks occurring between Northern Ireland and Great Britain, as she demands the two leave the EU as “one nation”.

The DUP leader told Mr Barnier "there is only one red line" as she refused to rule out her intentions to veto the Prime Minister's Brexit deal if it doesn't meet her strict requirements.

She told reporters in Brussels: "We’ve always said there’s only one red line in these matters.

"That’s when we’re treated differently from the rest of the UK in terms of customs in terms of regulatory alignment and that’s not only to protect the constitutional integrity of the UK but also the economy of Northern Ireland."

Prior to her meetings, she said: “We want the June 2016 referendum result to be respected and implemented. A majority of people throughout the United Kingdom voted to take back control of their money, laws and borders. Whilst many in Brussels may not agree with the referendum outcome, the result must be respected.

“As we leave the EU, we should do so as one nation. The United Kingdom single market must be protected with no new borders between Northern Ireland and Great Britain being created. From day one this has been the DUP’s only red line.”

Ms Foster rebuked claims that she may be ready to lend DUP support to checks, but only on goods moving from Great Britain to Northern Ireland.