A top Iranian cleric has issued a fatwa calling for Muslims to avenge the ‘blood of the martyr’ Ayatollah Ali Khamenei. 

Following the death of the Supreme Leader on Saturday, Iranian religious leader Nasser Makarem Shirazi delivered a formal religious decree that claimed all Muslims were obliged to avenge him. 

Shirazi stated that the US and Israel were the ‘main perpetrators of this crime‘.

A fatwa is an Islamic ruling made by religious authorities or scholars whose followers are compelled to follow. 

Experts have warned that the extreme proclamation means the world is more likely to see Iranian sleeper cells and lone wolves carry out horrific terror attacks at sites including embassies, restaurants and shops. 

Ghaffar Hussain, a counter-terrorism expert, told the Daily Mail: ‘Iran has a long history of exporting terrorism around the world and a track record of targeting dissidents and journalists in the UK.

‘This is a very real threat that has been heightened since the outbreak of the war and the one the security services must remain vigilant to.’  

In recent years, Iranian agents have been accused of attempting, and often succeeding, in carrying out bomb and assassination plots.

In 2018, France uncovered an alleged bomb plot that was set to be carried out at a Paris rally for Iranian opposition figures which was attended by 100,000 Iranians and hundreds of international dignitaries. That same year, Denmark accused Iranian intelligence of trying to assassinate an opposition figure on its soil.

And In 2012, Indian police concluded that Iranian agents were responsible for a bombing attack that targeted an Israeli diplomat in New Delhi, which wounded at least four people.

Iran’s supreme leader Ali Khamenei (pictured) was killed on Saturday in Tehran 

Iranian religious leader Nasser Makarem Shirazi (pictured) delivered a formal religious decree that claimed all Muslims were obliged to avenge the Ayatollah

Columns of smoke billowed over the skies of the capital, Tehran, following sustained Israeli-American bombardment on several cities across Iran on March 3, 2026

British intelligence agencies are currently closely monitoring Iranian sleeper cells, amid fears of retaliation over the US-Israeli attacks. 

Heiko Heinisch, a leading Islamism expert in Austria, told the Daily Mail: ‘We know that the Iranian Revolutionary Guards maintains functioning networks in Europe, primarily consisting of embassies, consulates, and mosques under the control of the Iranian regime.’

He warned that the most vulnerable targets were ‘soft’ ones ‘including companies, restaurants, or shops, that have a connection to Israel or are run by Jews.’ 

Heinisch added that Shirazi’s fatwa carried significant weight: ‘He is a well-known and respected legal scholar who was appointed to the Assembly of Experts for the establishment of the Iranian Revolution in 1979 and played a role in drafting the first constitution.

‘We should definitely take the fatwa he wrote seriously. A fatwa issued by a Grand Ayatollah like Shirazi can be regarded as an instruction.’

Last year, MI5 warned that Iran was behind 20 potentially deadly plots on British soil in the previous 12 months. 

Among Iran’s targets was the Israeli embassy in London, which sits just by Kensington Palace and Hyde Park. 

Other experts said that they believed that it was more likely that individuals with little to no connection to the Iranian regime, but who still believe in its values, pose more of a threat to the West.   

Avner Vilan, a former senior Israeli security official, told the Daily Mail: ‘What concerns me more are smaller-scale actions – either individuals who take the fatwa as inspiration, or regime supporters in Europe who may interpret it as a religious order and take matters into their own hands.

‘These may not necessarily be people directly working for Iran. They could be sympathisers or Shiite supporters of the regime who decide to act independently.

‘More organised operations, such as planting bombs or conducting complex attacks, require coordination and resources. At the moment, Iran is largely on the defensive, so I doubt they have the capacity to organise something like that right now.

‘What is more likely are sporadic acts carried out by individuals or very small groups. Even one person who believes they are following a religious order can cause havoc.

‘Sometimes, these actors are not even directly linked to the regime. They may simply see the fatwa, take it at face value, and decide to act on it independently.

‘We should also remember that different extremist groups can cooperate when their interests align. Even organisations that are rivals ideologically may work together if they share a common enemy.’

Dr Shmuel Bar, a former senior Israeli intelligence official, told the Daily Mail that organising sleeper cell activities takes significant time and resources, making the likelihood of an attack from a formal cell unlikely. 

He said: ‘Such operations take longer and Iran would now want to act more directly. Having said that, the awareness of Western intelligence to this option is high. The internet blackout in Iran will also make direct orders more difficult.’

Heinisch added: ‘Security agencies are keeping an eye on individuals who have ties to the Iranian regime. As far as I can tell, protection of Jewish, Israeli, and American institutions has already been stepped up across Europe. 

‘The problem lies with sleeper cells and lone perpetrators who have not attracted attention so far. Potential perpetrators can be found in groups that feel connected to the regime and in the radical anti-Israeli scene.’ 

As for what the fatwa may mean for Europe and the UK, he said it was ‘difficult to judge’.

He said: ‘The problem at the moment is that, on the one hand, there are too many potential perpetrators to monitor them all. On the other hand, there are too many potential targets. 

‘Obvious targets such as embassies and their employees or synagogues are under increased surveillance. But beyond that, there are simply countless soft targets. Since October 7, 2023, we have seen that for the anti-Israel scene – which includes supporters of the mullah regime, Hezbollah, and Hamas – virtually every recognisable Jew on the street and every Jewish business is a potential target.’ 

The academic pointed to previous instances of violence caused by fatwas. 

The most notorious was issued against British author Sir Salman Rushdie, who wrote ‘The Satanic Verses’ in 1989. 

The notorious book caught the ire of Ayatollah Ruhollah Khomeini, ruler of Iran from 1979 to 1989, who issued a fatwa against the book. 

As a result, Rushdie and his colleagues who helped spread the book around the world, including translators and publishers from Italy, Japan, Norway and Turkey, were injured or killed by religious extremists over the course of several decades. 

In 2022, Sir Salman himself was blinded in an attack at a literary festival in New York state. 

Heinisch said: ‘This shows that [Shirazi’s fatwa] could also have a long-term effect.’

He added that violence may have already taken place in response to the fatwa, pointing to the mass shooting at a bar in Austin, Texas, by a man who was wearing a sweatshirt that with the words ‘property of Allah’ on it, which the expert said ‘may have been motivated’ by the decree.