Haniyeh, who was in Tehran for the inauguration of Iran’s new president Massoud Pezeshkian, was instantly killed alongside his bodyguard Wassim Abu Shaaban.

He had been targeted by a bomb that had been smuggled into the guesthouse by Iranian agents hired by Israel’s intelligence and counter-terror agency Mossad, who were seen on closed-circuit television planting explosives in the residence, according to US and UK media. Explosive devices were placed in three rooms of the Tehran guesthouse, then one was detonated remotely from abroad.

The explosion shook the building, shattered some windows and caused the partial collapse of an exterior wall.

Hamas’s envoy in Iran Khaled Al-Qadoumi, who was in Haniyeh’s guesthouse at the time, told Iran’s Khabar Network he heard the explosion and he thought it was “an earthquake or thunder, lightning and rain”.

“But when I opened the window, the atmosphere was normal,” he said. “Moments later, I noticed smoke rising and felt that something had happened.

“Shortly after, Iranian protection told me that Ismail Haniyeh was martyred. As a representative of Hamas in Iran, I asked to see Haniyeh and, when I entered his suite, I found the ceiling and walls from the outside collapsing on his body from the outside to the inside, and the same situation was also with Haniyeh’s companion, the martyr Wassim.”

Mr Al-Qadoumi’s description seems at odds with Iran’s official explanation of the blast, which Tehran claims was an airborne attack in which a “short-range projectile” weighing about 7kg was fired at the building causing a “strong blast” that killed Haniyeh and his bodyguard.

Khalil al-Hayya, a Hamas official, told a press conference in Tehran: “A missile hit the room where Haniyeh was in, and the windows broke and the wall collapsed before it hit.”

It is believed that planning for the assassination took months and required extensive surveillance of the compound.

The original plan was to assassinate Haniyeh in the building in May when he attended the funeral of Ebrahim Raisi, Iran’s former president, according to the UK Telegraph. Raisi died in a helicopter crash.

The operation didn’t go ahead due to the large crowds inside the building, two Iranian officials of the Revolutionary Guard Corps said.

“Instead, the two agents placed explosive devices in three rooms of the Islamic Revolutionary Guard Corps guesthouse in north Tehran where Haniyeh might stay,” the Telegraph reported.

The Iranian news website Event 24 reported that Haniyeh “was housed completely anonymously” in the north of Tehran, and even the janitor of the building and the local police did not know about his identity.

“Except for the heads of the forces, no one knew about the residence of Haniyeh, except [for Mohsen] Haji-Mirzaei, the head of the President’s office, who was informed about the identity of the person who was accommodated … for the necessary security arrangements,” it says.

Israel has not publicly acknowledged responsibility for the killing, but Israeli intelligence officials briefed officials from the US and other Western governments on the details of the operation in the immediate aftermath, in a move that “surprised and outraged” US officials.