Tory former Cabinet minister Lord Chris Patten has said he does not want Northern Ireland to go back to the days of "shooting and maiming" as he urged the Government to be "very careful" over how Brexit impacts on the border.
The ex-EU commissioner made the call as he delivered a scathing assessment of how withdrawal will hit Britain if it leaves the customs union.
Lord Patten, who chaired a commission into policing in Northern Ireland as part of the peace process, said: "I feel emotionally very strongly about this. I think we did a really good job on the Good Friday Agreement. And I don't want to go back to the days when people were being shot and maimed.
"There is a simple answer to the Northern Ireland border question and to much else besides. Let's stay in the customs union."
The comments came as Lord Patten traded jibes with arch-Brexiteer Jacob Rees-Mogg, who likened opponents of withdrawal to Japanese soldier Hiroo Onoda, who spent 29 years in the jungle because he did not believe the war was over.
Mr Rees-Mogg, who chairs the Conservative pro-Brexit European Research Group, said reneging on the referendum result would be a national humiliation worse than the Suez crisis.
He said: "With one year to go before the technical date of departure, this is the challenge to the decreasing number of Remainers who model themselves on Mr Hiroo Onoda, the Japanese soldier who finally surrendered in 1974 having previously refused to believe that the Second World War had ended."
Lord Patten hit back saying Brexiteers lacked the experience to negotiate a trade deal, stating: "One problem is that the ministers who talk about these fictitious trade deals have never negotiated one.
"The closest they have come to a trade deal is the checkout at Waitrose."
Lord Patten also took a swipe at Mr Rees-Mogg, who he said he has known since he was eight, branding him a "caricature".
The peer said: "The thing about Jacob is that he is much better than he has allowed himself to seem.
"It is all very well having those views and being thought to be rather an eccentric but interesting lad when you are eight. But having the same views when you are 48 raises, I think, one or two eyebrows.
"And, I think, he has allowed himself a bit to be taken over by his own image and caricature."
Mr Rees-Mogg said he did not want to "lay into" Lord Patten as he was an old family friend, before adding: "I will only say this, when he was governor of Hong Kong I greatly admired his efforts to defend and promote the rights of the people of Hong Kong and to argue for democracy.
"It is a great sadness that he wanted democracy for the people of Hong Kong but rejects it for the people of the United Kingdom."
Meanwhile, prominent pro-Europe Labour MP Chuka Umunna launched a strong attack on his front bench for "parroting" the Tories on immigration.
He said: "One of the things I cannot abide, and I find intensely irritating is, frankly, to hear the Labour front bench increasingly parrot what the Conservative front bench says about this issue - immigration and Brexit - and, increasingly, too many other issues.
"You need only look at the comments of Liam Fox's shadow, Barry Gardiner, in respect of the Brexit issue, and you do wonder where is the Opposition?"