The UK's chief Brexit negotiator has been summoned to appear in front of the Northern Ireland Affairs Committee to answer questions about how he intends to resolve issues over the Irish border.
David Davis' Parliamentary Under-Secretary in the Brexit Department has already appeared before the committee of MPs - but they said the issue was too important and required a minister to attend.
The committee has also met with chief EU negotiator Michel Barnier. Mr Barnier, who gave evidence to the committee in January, this week completed a two day visit to Northern Ireland during which he met with residents and business leaders in the border towns of Newry and Londonderry.
Mr Davis visited the border for the first time as Brexit Secretary on April 23 when he spent two hours at a disused customs post and an autism centre in Middletown, Co Armagh. He was strongly criticised for not announcing his visit beforehand.
In a letter to Mr Davis, the chairman of the committee, Dr Andrew Murrison MP, said: "The issue of the border remains unresolved.
"Finding a solution is key to securing the right deal for the UK as we leave the EU.
"To date we have not seen a worked up coherent proposal that would allow the UK to be freed from the strictures of the Customs Union and Single Market and avoid additional infrastructure at the border." The letter also welcomed the evidence it had heard from the Parliamentary Under-Secretary, Robin Walker MP, but said that "given the importance of the border to the UK-EU negotiations, we believe we need to hear from you as the minister leading the talks".
It added: "We welcome your recent visit to the border in Northern Ireland. Visiting the border is crucial in understanding the complexity of the problem and we are glad you have had the opportunity to see it. You may wish to use the evidence session to discuss the visit and how it is informing your work in developing solutions."