The first minister of Wales, Carwyn Jones, has branded the pursuit of a hard Brexit “a kind of religious fundamentalism”.

Jones also compared Westminster’s approach to the devolved nations over Brexit to the attitude of the Soviet leader Leonid Brezhnev to the eastern bloc.

In an interview to mark the 20th anniversary of the referendum that led to Welsh devolution, Jones told the Guardian he believed that Wales was more successful and confident than two decades ago.

He said, however, that Brexit was creating challenges for both devolution and the union.

The people of Wales voted to leave the EU, but Jones said the Tories’ poor showing at this summer’s general election across the UK showed citizens did not want a hard Brexit.

“There were a lot of people who voted leave for reasons that have nothing to do with the EU,” he said. “A lot of people said to me on the doorstep: ‘We’re going to kick David Cameron. Don’t worry, we’re still Labour.’ I heard that more than anything else. The EU was a minor issue in the referendum about the EU.”

Jones, who has been first minster since 2009 and is the leader of Welsh Labour, said that when people voted for devolution in the 1997 referendum the detail of how it would work had been published.

“People could see what it would look like,”he said. “The problem with the [EU] referendum was that the vote was to leave, but there was any number of interpretations over what that meant.

“The narrative of the hard leavers is that the only interpretation you can put on this is a hard Brexit. It’s a kind of religious fundamentalism. This is the only true way. Nobody can disagree with us. That narrative has to be challenged. People were offered the chance to vote for a hard Brexit in June and didn’t.”

The Labour-led Welsh government is working with the SNP in Scotland to fight what the two administrations sees as a “naked power grab” by Westminster in the Brexit bill. The UK government has said the devolved administrations will be asked to consent to the bill.

“They have said they want the consent of the devolved legislatures in order for the bill to go forward. We welcome that,” but “they can’t say: ‘Yes you can take the decision as long as you agree with us,” Jones said. “That isn’t consent, it’s that’s the doctrine of limited sovereignty from the days of Brezhnev.”

He said the Welsh government had to make sure Brexit did not interfere with the delivery of the public services it runs such as health and education.

“People want to see improvements in the health service, our GCSE results improving. They want to see the school building programme continuing, but Brexit sits there in the background,” he said.

“We can’t be prosperous unless we have the fullest possible access to the single market. That’s hugely important for us, 67% of our exports go there. Any obstacle and we lose jobs. It’s that simple. We don’t have to have the sort of Brexit that the Tories are proposing.

“The difficulty with the Westminster government is that I have no idea what their view is. They don’t have a collective view. There are different views held by different ministers. The fear I have is that the UK is not seen as a serious player anymore, and that’s bad for all of us who live in the UK. As for the prime minister, she’s not visible.”

Asked if this was a dangerous moment for the future of devolution and the union, Jones said: “I think there are great challenges for the UK. To my mind the EU was part of the glue that held the UK together.

“As someone who wants the UK to stay together, I don’t want daft decisions such as the current bill to cause the UK to begin to unravel. I’m a devolutionist who believes in the union. I don’t think they get that in Whitehall.

“I think there are some of them who take the view the UK is the same as it was in 1972, a unitary state with one government. They have not got devolution for the last 20 years, and there is danger in that. We can’t allow people to think the choice is between a centralised state and independence.

“There is a wiser and more sensible choice, which is devolution in the UK context. Does that mean federalism? It means something close to it. Federalism is difficult because of England. What do you do with England? But it means that the UK is a partnership of four nations, not one that imposes its will on everyone else.”

It is ironic that concerns about Brexit have led to Jones, a champion of the union, working with the SNP, which campaigns for independence.

“There’s no reason why the union can’t stay together,” Jones said. “There is every possibility of creating a modern democratic state where there are different centres of democratic accountability rather than one, London.”

He summed up the impact of devolution on Wales in one word – confidence.

“Back in the nineties, people in Wales completely lacked confidence. At the time of the referendum people said to me: ‘It’s a nice idea but we’re not really capable of it” as if we were completely stupid and couldn’t take our own decisions,” he said.

“That affected people’s views of themselves. Young people in Wales didn’t see a future for themselves in Wales. Now they stay here. They can see their future here. There are opportunities here that didn’t exist in the nineties.”