Former US president Jimmy Carter has breached diplomatic protocol by making specific public claims about Israel’s nuclear arsenal.

He became the first president to acknowledge publicly Israel’s nuclear capability when he addressed an invited audience of journalists last week at the Hay-on-Wye literary festival, on the Welsh borders.

After describing Israel’s blockade of the Gaza Strip as “one of the greatest human-rights crimes on earth”, he declared that Israel had more than 150 nuclear weapons.

This was in response to a question about how a future US president should deal with the Iranian nuclear threat.

Listing international nuclear capability, he said: “The US has more than 12,000 nuclear weapons, the Soviet Union has about the same, Great Britain and France have several hundred, and Israel has 150 or more.”

Although the existence of Israeli nuclear arms is widely assumed, Israel has rarely, if ever, referred to them and American officials have stuck to that line for years.

This week, Israel’s press spokesman, Mark Regev, steadfastly refused to comment.

Mr Carter, 83, told the festival audience that the West should begin talks with Iran immediately, “beginning at a low level, working up to foreign ministers and then at a presidential level.

“We need to let them know both the benefits and the detrimental side of continuing their nuclear policy.”

He cited Iran’s position as a signatory to the Nuclear Non-Proliferation Treaty and emphasised that Israel remained one of the only non-signatories. “Iran has a right to nuclear enriched fuel. America has to say ‘we want to be your friend, we want communication and trade with you, we want to give you adequate technology and fuel to build your peaceful nuclear programme’. I think that’s the best approach.”

Citing the Iranian crisis during his own presidency in which 52 American diplomats were taken hostage in the US embassy in Tehran, Mr Carter said that he had not refused to talk “just because I disagreed”.

When asked whether Europe and America ought to be negotiating with Hamas, he said: “Israel is doing business with Hamas every day. Everyone in their right mind knows you can’t have peace without talking to Hamas.”

Europe, he said, should encourage the formation of a unity government to include Hamas and Fatah. He referred to the current Palestinian government as “a subterfuge”.

Citing opinion polls that showed increasing support for Hamas, he claimed that Palestinian president Mahmoud Abbas was perceived as having “sold out to the Israelis and Americans”, while Hamas was “more fervent in protecting Palestinian rights”.

But as for Hamas recognising Israel, Mr Carter thought this was impossible “unless Israel is willing to recognise Hamas. You’re asking for a radical group to give diplomatic recognition to Israel, occupying all of their land, when Israel has no intention of recognising Hamas.”

Describing the “complete” influence of the pro-Israel lobby groups in America, Mr Carter said: “It is politically unacceptable for anyone holding US public office to criticise the state of Israel.”

Mr Carter also complained that the European Union’s lack of criticism of Israel’s blockade of Gaza was “embarrassing”.

He said the EU should establish relations with Hamas if the group agrees to a ceasefire in Gaza.