A shadow cabinet minister who described the Good Friday Agreement as a shibboleth has said he is deeply sorry that remarks he made about the deal had led to a "misunderstanding".
Barry Gardiner insisted he "absolutely" does not believe the agreement is outdated and said it was a vital part of the relationship between the UK and Republic of Ireland.
The shadow international trade secretary had been criticised by a Labour former Northern Ireland Secretary after a recording of the comments emerged.
Lord Hain told the Press Association Mr Gardiner was "unbelievably ignorant and irresponsible".
"The Good Friday Agreement has delivered 20 years of peace now being relentlessly undermined by the Tory Government dogmatists putting a hard Brexit before that peace," he added.
Labour former shadow Northern Ireland secretary Owen Smith said the remarks were "reckless".
Former prime minister Tony Blair told BBC Radio 4's Today programme: "I don't know how anyone can say that.
"It's the only basis upon which you're going to have peace," he added.
Shadow Brexit secretary Sir Keir Starmer said: "Labour fully supports the Good Friday Agreement in all its aspects, including no hard border. Crucial this is fully respected in the Brexit negotiations and beyond."
The row comes on the 20th anniversary of the Good Friday Agreement.
Mr Gardiner was heard in a recording suggesting the border issue and the agreement had been "played up" and described the deal as a shibboleth, a Hebrew term used to describe a long-held custom that is outdated.
The shadow international trade secretary said it was "hugely" in the Republic of Ireland's economic interest to make sure there is no external border.
In the recording of an event in Brussels in March, obtained by The Red Roar website, he was heard saying: "We must also recognise there are real economic reasons why people have played up the issue of the Irish border and the need to have the shibboleth of the Good Friday Agreement.
"That is because it is hugely in the Republic of Ireland's economic interest to make sure there is no tariff and no external border there."
In a statement on Tuesday, Mr Gardiner said: "The Good Friday Agreement is a vital and essential part of the relationship between the UK and the Republic of Ireland, and has been central to the two decades of peace it has brought about.
"Labour is completely committed to the agreement and opposed to any return of a hard border between North and South.
"We are committed to negotiating a new customs union between the UK and Ireland as part of a final Brexit settlement, which would play a key role in ensuring there is no hard border.
"I am deeply sorry that my informal remarks in a meeting last month have led to misunderstanding on that point - in particular, that my use of the word "shibboleth" in its sense of "pass word" or "test of membership" gave the impression that I thought the Good Friday Agreement was in any way outdated or unimportant. I absolutely do not."