Nuclear weapons were once deemed unIslamic by Iran’s supreme leader who went as far as issuing a fatwa against their production.

But a lot has changed since the 1990s, and although Ayatollah Ali Khamenei still sits atop of the regime, nuclear arms appear to be far from taboo.

Tehran, of course, insists it has no plans to weaponise its nuclear programme, which it claims is for civilian purposes only.

And the latest assessment by US intelligence is that Iran “ is not currently undertaking the key nuclear weapons-development activities necessary to produce a testable nuclear device.”

But that isn’t the whole story.

Iran is ramping up its production of highly-enriched uranium, just short of the weapons grade needed for an atomic bomb, at the underground Fordow Fuel Enrichment Plant.

Since 2019, and as of February, Iran has increased its amount of enriched uranium from 997kg to 5,525kg.

This includes an increase in uranium enriched up to 60 per cent, or near “weapons grade”, from 88kg to 123kg in the past year, a 38 per cent increase.

UN inspectors noted this in a February report, which warned of new equipment and expansion at Fordow, which is a former Islamic Revolutionary Guard Corps base and 20 miles northeast of the city of Qom.

“Iran is sitting on the threshold of nuclear weapons; it can build a bomb more quickly than at any point in its history,” said Kelsey Davenport, director for Nonproliferation Policy at the Arms Control Association.

“If Tehran makes the political decision to develop a nuclear arsenal, it can produce enough weapons-grade uranium for an explosive device in less than a week and enough for five or six weapons in a month.”

She added, “Building a bomb would take more time – likely six months to a year – but that process will take place at covert, undeclared sites, making it more difficult to detect and disrupt.”

Iran’s nuclear deal

The roots of this build-up lie in the collapse of the Joint Comprehensive Plan of Action (JCPOA), known as the Iran nuclear deal.

Signed in 2015, it offered Iran relief from Western sanctions in return for curbs to and international monitoring of its nuclear programme. Parties to the agreement included the UK, the EU, the US, and China and Russia.

The Iran deal was hated by US conservatives. Donald Trump, then campaigning to replace Barack Obama as president, branded it “the worst deal ever”.

In 2018, despite the entreaties of his Western allies, the then President Trump pulled the US out of the JCPOA and restored Washington’s sanctions against Tehran.

The move mortally wounded the JCPOA. Its ghost lumbers on in the form of some UN inspections of Iranian facilities.

An effort to revive the agreement early in Joe Biden’s presidency failed in the face of Iranian resistance and a lack of political will in the US.

Tehran has asserted its right to cancel parts of the deal since the US withdrawal.

Under the pact, Iran had agreed to tight limits that ensured it could not stockpile enough enriched uranium to produce a single bomb.

Enriched uranium numbers are still significantly below 2015 levels, the year the JCPOA was signed, when they hit 7,953kg but are increasing at a similar rate to that seen in the early 2010s.

UN inspectors have not been able to access some crucial facilities in the nuclear programme, including centrifuge production workshops, since February 2021.

On Sunday, Iran barred inspectors on the grounds of “security considerations” over a possible Israeli attack in retaliation for its missile and drone assault on Israel on Saturday.

“The bottom line is that Iran’s nuclear programme has advanced pretty dramatically and with far less international oversight than at any point in its history since the US withdrawal from the JCPOA,” said Julien Barnes-Dacey, of the European Council on Foreign Relations.

“Effectively today everyone’s flying blind. In less than a year’s time, the world may well be contending with a nuclear Iran,” said Urban Coningham , of the  Royal United Services Institute think tank on defence.

Five years after the JCPOA withdrawal and three years after Joe Biden’s attempts to renegotiate failed, he said there was “still no new methodology for engaging with Iran”.

What’s stopping Iran?

Tehran calculates that it stands to gain more in terms of sanctions relief and future negotiations by dangling the threat of the bomb than actually building it.

It has carefully cultivated a network of proxies such as Hezbollah and Hamas across the Middle East as part of a “forward defence” strategy to contain Israel.

Iranian officials have begun talking of their “deterrent”, jargon associated with nuclear bombs, and suggesting that they have the ability to build a bomb when they want to.

Weaponising would be dangerous. Far better instead to leverage the status of a threshold nuclear power.

Escalation carries significant risk to the regime, which explains why the April 13 attack was so clearly telegraphed and more performative than intent on serious damage.

But if Israel retaliates, Tehran could decide it needs the bomb for its own security and has already shown a taste for danger.

“While the recent Iranian attack was ultimately ineffective, it illustrates that the regime in Tehran is deeply reckless, and seemingly comfortable with the risk that one drone or missile strike could have hit a densely populated urban target and completely changed Israel’s response,” Mr Coningham said.

“Iran will pay a steep price for developing nuclear weapons, so it will not make that decision lightly,” Ms Davenport told The Telegraph.

She added: “Escalating tensions between Israel and Iran increases the risk of Tehran determining that nuclear weapons are necessary for its security, particularly if Israel responds to the April 13 attack with a counterstrike on Iranian territory.”

Mr Barnes-Dacey said, “The intelligence suggests they don’t want to weaponise at the moment but that could change on a dime.

“Iran could decide, particularly given the worsening regional situation, that actually ultimately a nuclear deterrent is precisely what it needs in the context of a more aggressive Israel and the prospect of Trump coming back into power.”

Axis of Putin and Trump

The prospect of Mr Trump winning November’s US elections is not the only factor in play in a complicated geopolitical picture that also involves Russia.

Vladimir Putin has deepened Russian ties with Iran since his illegal invasion of Ukraine made Moscow an international pariah, which risks emboldening Tehran.

Iranian president Ebrahim Raisi told the Russian president in a call on Tuesday that Tehran’s strikes on Israel were limited and that the Islamic republic was not interested in escalation.

Russia won’t now play the same role it did in the negotiation of the JCPOA, Mr Barnes-Dacey said.

“The prospect of a kind of unified international coalition, including Russia and China, coming together to squeeze Iran on the nuclear issue is pretty non existent at this stage,” he said.

An already unpredictable situation is complicated by the age of Iran’s Supreme Leader. When the 84 year-old dies, his fatwa could be reshaped by a more hardline successor.

“It could well be that hardliners are able to seize the reins of power more fully and push the nuclear agenda forward without really caring about making any deal with us,” Mr Barnes-Dacey said.

Can the West stop Iran?

Western influence is at a low ebb with the region in such turmoil and the chances of the JCPOA being resurrected appear slim to non-existent.

Joe Biden was captured on video describing the deal as “dead” as long ago as December 2022.

“While the JCPOA is off the table, diplomacy is not. It is past time the United States puts a deal on the table to stabilise the nuclear crisis and prevent further escalation,” Ms Davenport said.

“A package that includes enhanced transparency in exchange for limited economic relief would be a good place to start.”

But with the Iran deal a culture war totem, there is no political will to engage in sanctions relief with Tehran.

Mr Barnes-Dacey said: “At the moment, that seems impossible given US domestic politics and the forthcoming election. 

“Iran is now one of those one of the poisonous issues where there’s no room for manoeuvre in the US.”