THERESA Villiers has come under further pressure to explain why she is so confident that the UK-Republic of Ireland common travel area will continue to operate in the event of a Brexit.
The eurosceptic secretary of state told The Irish News in February that there need not be any restrictions on the movement of people and goods across the border should the Leave campaign be victorious in the June 23 referendum.
However, she has since been contradicted by key figures on both sides of the EU debate and by the Irish government.
Ms Villiers's cabinet colleague David Lidington has become the latest to question the Chipping-Barnet MP's assertion.
During a visit to Northern Ireland yesterday, the British foreign minister said the secretary of state's claim "flies in the face of reality".
Pro-EU Mr Lidington said he agreed with Irish foreign affairs minister Charlie Flanagan, who last week told The Irish News that the fate of the post-Brexit border would be decided by the remaining members of the EU and not by the governments in Britain and the Republic.
"Any British government would want to try to maintain the common travel area but where Charlie Flanagan is right is that this would depend on what Ireland's obligations were with an EU of 27," Mr Lidington said.
"What you cannot say is that there is no risk to the common travel area – that, I think, flies in the face of reality."
The British minister said he respected Ms Villiers's position but "some serious questions" about cross-border trade and movement would arise if the UK were to leave the EU.
"It would all depend on the nature of the agreement that could be reached governing the future relationships, and whatever rules the EU at 27 decided should apply to countries with an external border," he said.
"If we (the UK) were not part of the customs union with the EU then there would have to be customs checks at the border."
Mr Lidington also claimed Ms Villiers's position highlighted the contradictions of pro-Brexit campaigners.
"What I find puzzling is the supporters of Leave say they want to stop free movement, yet if they assume that the Irish border will be completely open then that is allowing a back door, which defeats the very policy they want because there would still be complete freedom of movement into the the Republic of Ireland from elsewhere the EU, so how's the Leave campaign going to have this situation?
"This is another illustration of the lack of coherence in the Leave campaign's position."