John Bruton (left) and Bertie Ahern give evidence to the committee at Westminster yesterday

The ex-European Union ambassador to the US said he does not envisage passport checks on routes in and out of Northern Ireland once the UK completes the split.

But in response to a direct question at the House of Lords EU Select Committee on whether Brexit could be abused and become a smugglers’ charter, Mr Bruton said: “Yes is the answer to that question too.”

Mr Bruton joined former taoiseach Bertie Ahern to set out their biggest concerns for the Irish and Northern Irish economies and communities post-Brexit.

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A recent suggestion that Irish ports and airports would become proxy points of entry into the UK and a type of frontier for British immigration checks was dismissed out of hand by Mr Ahern.

“I quite frankly just found that unbelievable,” he said.

“To put that suggestion forward is a total lack of understanding of how people think, north and south, of either tradition. It just would not happen.”

The idea was floated in recent weeks with NI Secretary James Brokenshire saying that governments in London and Dublin would work to strengthen the Republic’s external borders after Brexit.