Former UK prime minister Sir John Major has criticised the “breathtaking ignorance” unionist leaders and Eurosceptics have shown over the Irish border during the Brexit debate.

Sir John called for calm but warned that the imposition of a hard border was a possibility as a result of Theresa May’s decision to defer a parliamentary vote on the withdrawal agreement her negotiators reached with the EU.

Delivering the inaugural "Albert Reynolds Memorial Lecture" in Longford on Monday, Sir John described claims made by hardline Brexiteers over how any backstop would threaten the union going forward as “wholly misguided”.

“Some opinion including many who believe themselves to be unionists has shown a breathtaking ignorance,” he said during the lecture, which seeks to recognise the contribution Reynolds made to the Northern Ireland peace process.

“To them, Irish demand for a backstop is a bogus ploy to keep the UK in the customs union. In truth, a backstop is a vital national interest for Ireland and the UK.”

Sir John and Reynolds have been credited with setting the peace process in motion through their co-signing of the December 1993 Downing Street Declaration.

“If the House of Commons cannot find a way forward then the risk of a hard border remains possible,” he said.

Sir John said a “reckless few” had failed to see the likely ramifications that a hard border would bring and that the challenges facing Mrs May’s administration were only beginning.

“Even if at some date the prime minister’s plans are approved the problem is not solved,” he said. “A hard border, now or at the end of a long transition period or at any time would be disastrous. Peace isn’t secure, it never is and any new border would be a focus for the wild men on the fringes to reactivate old disputes and hatreds that should be laid to rest forever.”