There are concerns UK companies such as Tesco could "write off" Northern Ireland once the Irish Sea border is in place, a peer has claimed.
Ministers yesterday defended a lack of information about what the new sea border will look like.
Six months after Brexit the specifics of how goods moving between Northern Ireland and Britain will be treated have still not been agreed. But NIO minister Robin Walker told a Lords committee that "we cannot necessarily provide all the detail that everyone would like on day one".
Lord Kerr asked Mr Walker at the Lords EU committee: "How confident are you that GB businesses are just not going to write off the Northern Ireland market?"
He added: "I am very concerned. Are you talking to the big stores?
"Are you talking to the Tescos who have an operation in Northern Ireland but all the extra costs of this, and all the [extra] bureaucracy, may make it to them seem just not worthwhile?"
The implementation of the Northern Ireland protocol is subject to negotiation by a joint committee between the EU and UK.
The UK argues it should not have to apply some customs controls it agreed to as part of the agreement - such as exit summary declarations for goods moving across the Irish Sea.
The dispute means that businesses on both sides of the Irish Sea still have no detail about what will happen to the border in five months when the transition period ends at the close of the year.
Mr Walker told peers: "I absolutely recognise that business will always want as much detail as possible as early as possible, and that this is a process where we cannot necessarily provide all the detail that everyone would like on day one.
"We do have to ensure that the voice of business is heard and fed into the process, and we also have to make sure we don't give any misleading information in terms of things that haven't been fully bottomed out and agreed.
"There are elements of the approach to the protocol which still require agreement at the joint committee and it would be wrong for us to imply that those were fully decided."