Northern Irish Democratic Unionist Party (DUP) leader Arlene Foster holds a news conference at the European Parliament after a meeting with EUÕs Brexit negotiator Michel Barnier in Brussels October 9, 2018. REUTERS/Yves Herman

BRUSSELS (Reuters) - The leader of Northern Ireland’s biggest party on Tuesday ruled out any checks on trade with the British mainland after the EU’s Brexit negotiator tried to persuade her that such controls could be “de-dramatised”.

Arlene Foster, whose Democratic Unionist Party props up Prime Minister Theresa May’s government in London, said the DUP would never accept the regulatory or customs controls that the EU’s Michel Barnier says are an essential “backstop” to avoid a hard land border with EU member Ireland after Britain leaves.

“There is only one red line,” Foster told reporters after her meeting with Barnier in Brussels, as he presses to secure a deal on the Irish border issue that can unlock a withdrawal accord with London in time for a summit next Wednesday.

“We could not support any arrangement which could give rise to either customs or regulatory barriers within the UK internal market,” Foster said, adding that she looked forward to further meetings in Brussels over the coming two days.

Barnier tweeted after their meeting that he was “working hard to explain and de-dramatise the backstop” -- a text by which the EU proposes keeping Northern Ireland inside the EU economic space in the event that a future EU-UK free trade deal fails to find a way to avoid disruptive checks on their land frontier.

EU negotiators have floated ideas about making customs and regulatory checks on goods by discreet technical means well away from sea transit routes between Britain’s mainland and its Irish province to try to overcome objections from Foster -- and May -- that such differences undermine the United Kingdom’s integrity.

Asked about that push to “de-dramatise” checks if the backstop were triggered, Foster said: “The whole point of checks is there’s a difference. Why would we need checks ... if we were an integral part of the single market of the United Kingdom?”

She said she had yet to see new proposals that May plans to put to Barnier in a bid to address the problem.

Pressed on whether the DUP was prepared to block a Brexit deal and so pitch Britain into a “no-deal” scenario next March, Foster said she believed her party was not alone in its view, and that she expected May to respect its position.