The UK made a "colossal mistake" when it left the European Union, former prime minister John Major has told a Westminster committee.
He said while he is not a "significant Europhile", he believes the UK was stronger in the EU.
The former PM was appearing before the Northern Ireland Affairs Committee, which is investigating the effectiveness of the institutions of the 1998 Good Friday Agreement.
Alliance Party deputy leader Stephen Farry asked if it was an advantage for the UK to be in the EU during the early days of the peace process in Northern Ireland in the 1990s.
Mr Major, who campaigned to remain in the EU ahead of the 2016 Brexit referendum, said membership of the bloc "smoothed the way" for many of the agreements at the start of the process.
A lot of business was conducted on the fringes of EU meetings, he said, adding that "it was a very useful tent in which we could operate discreetly.
"We used that tent to the full," he said.
"Ireland are our nearest neighbours. That relationship with Ireland mattered then and it matters now," Mr Major said.
"We are out of the European Union and they are in it.
"They are one of the ways we may stretch out and recover some of the things we so wilfully threw away when we left the EU."
Mr Major said he does not believe the EU is anything close to perfect.
"Because I negotiated the Maastricht Treaty, the ultra Brexiteers regard me as a significant Europhile.
"That is not actually true. I am a very practical European. There are many things I don't like. But I look at the package and I say, 'Are people in my country better off and safer if we were inside the European Union or if we were outside?'
"I reached the conclusion we would be better off inside.
"There are three great power blocs in the world today. [The] United Kingdom is not one of them. There is America, there is China and there is the European Union.
"Europe is going to face immense economic competition from both America and China. It may face military insecurity problems with China.
"Is Europe better able to represent its people with the United Kingdom inside making it stronger or outside making it weaker?
"I think not just us but the whole of Europe is stronger with Britain in it."
Mr Major added: "When I look at the interests of my children and my grandchildren, I think their future is brighter if they are part of a really big bloc who could work with us if we are in difficulty.
"Suppose we had a socking great row with China. Britain decided to put sanctions on China; China wouldn't be much bothered.
"If the European Union did because we were being maltreated, they would feel quite differently about it.
"It is those strategic issues which make me believe we should be in Europe and that we have made a colossal mistake in leaving.
"I understand the sovereignty arguments, though many of them are more semantic than real. No country, not even the United States, has pure sovereignty.
"Who has pure sovereignty in Nato? We all sign up to Nato; nobody complains about that.
"I wonder how many of the cuts we have had, or the shortages for our public services, would not have happened but for the loss of GDP because we have left the European Union?"