A senior German MEP and ally of Chancellor Angela Merkel has warned the UK Government that the EU could use Northern Ireland as a bargaining chip in the Brexit negotiations.
As peers prepared for battle with ministers over the Article 50 Bill in the House of Lords, Elmar Brok warned Theresa May to be careful about the tactics she chooses to employ in talks.
He told the Guardian newspaper: "The British Government tries to divide and rule. They believe they can take members of parliament out of certain nations ... to win support by dividing us.
"If they try to negotiate while trying to interfere in our side then we can do that too.
"We can make a big fuss over Scotland. Or Northern Ireland."
Later, Mrs May ramped up pressure on peers over Brexit by attending the opening of a crucial debate in the chamber of the House of Lords yesterday.
In a highly unusual move, the Prime Minister sat on the steps in front of the Royal Throne as Lords leader Baroness Evans of Bowes Park urged peers not to frustrate the passage of a Bill which will give Mrs May authority to launch EU withdrawal negotiations under Article 50.
Her presence, in a position she is entitled to occupy as a member of the Privy Council, was seen as a visual warning to peers not to seek to block or delay the European Union (Notification of Withdrawal Bill) in the Upper House, where the Government does not enjoy an in-built majority.
Speaking during a by-election campaign visit to Stoke ahead of the debate, the Prime Minister said peers should "pay attention" to the fact that the Bill was passed unamended by a large majority of MPs in the House of Commons. And she added: "Properly there will be debate and scrutiny in the House of Lords, but I don't want to see anybody holding up what the British people want, what the people of Stoke-on-Trent voted for last year, which is for us to deliver Brexit, to leave the European Union."
Around 190 peers are expected to speak during the two days set aside for the Bill's second reading, the first opportunity for the upper chamber to debate the legislation. No votes are expected during second reading, but the Government is braced for a battle over EU citizens' rights and a meaningful parliamentary say on the final Brexit deal when the Bill returns for its committee stage next week.
Opening the debate, Lady Evans said she was "confident" that peers would take a "constructive approach".
Labour's former Northern Ireland secretary Lord Hain said the Government must commit to a completely open border between Northern Ireland and the Republic of Ireland.
He said: "A One Nation Brexit would also mean guaranteeing a completely open border between Northern Ireland and the Republic.
"No security checks, no controls physical or electronic, otherwise I believe the peace process could unravel."
Ulster Unionist Party peer Lord Empey agreed with Lord Hain on the importance of an open border.
Lord Empey said: "I would, however, caution him not to close the door on electronic or other technical mechanisms because that could avoid having to have a physical border which would be a major, major set back for all of us."
Lord Empey also cautioned against linking the Belfast Agreement to Brexit.
He said: "There are political issues, I do not doubt, but there is no legal link."