The return to a hard border would be a "political failure" by both the British government and the European Union, a former head of the British Army has said.
General Lord Richard Dannatt was commissioned into The Green Howards regiment in 1971 - he served in the north and received the Military Cross at the age of 22.
His comments come days after reports that Brexit talks have run into a "significant problem" over the issue of the border.
EU negotiators are said to be demanding a "backstop to the backstop" to prevent a return of a "hard border" between the north and the Republic.
British prime minister Theresa May has insisted that any backstop must be time-limited, but the Dublin government has already ruled this out.
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Asked by the Press Association if he was concerned that a hard border could result in a return to the Troubles, Gen Dannatt said: "I don't want to see a return to a hard border.
"It would be a political failure by our own government and by the European Union 27 if that was to be the case.
"Over 650 British servicemen lost their lives over the 38-year campaign to produce the much more harmonious society that there is in Northern Ireland now.
"I don't want to see politics bringing a regression and the return to bloodshed.
"I don't think it will happen, I don't want it to happen, it would be a disaster if it did happen, it must not happen."
Theresa May has insisted an exit agreement is still "achievable" despite negotiations stalling just days ahead of a crunch summit in Brussels tomorrow.
Asked for thoughts on how veterans are treated by the British government, Gen Dannatt said more is being done to look after them than in the past, but this is still not enough.
He served as chief of the general staff between 2006 to 2009 and recently revealed that he had been questioned by two investigators from the PSNI about an event in 1973, on his last day as head of the British Army.
He said he had no difficulty in telling them what had happened, but that he is "particularly concerned" about looking back at the Troubles.
"There are a small number of cases where there are soldiers in their late 60s, 70s and early 80s who are living in fear of a knock on the door, and that is wrong," he said.
"That has got to be gripped, and I think the sooner it is gripped the better. The pressure will continue to mount on the government.
"I understand the prime minister told the attorney general to sort it out - come on, mate, sort it out."
