Andy Burnham said Iran’s Islamic Revolutionary Guard Corps were “almost certainly” behind threats to RAF Fairford, the firmest attribution of responsibility made so far by the UK after an incident late last month.

The prime minister told MPs the UK would bring further sanctions against Tehran and the IRGC in the aftermath of an ambiguous episode that, despite over a fortnight of police investigation, has yet to lead to anybody being charged.

Addressing parliament on Monday about events at RAF Fairford, Burnham said: “I can confirm our assessment that it is highly likely to have been coordinated in Iran, almost certainly directed by Iran’s Islamic Revolutionary Guards.”

In a statement to the House of Commons, the home secretary, Shabana Mahmood, told MPs that the foreign secretary, Ed Miliband, had summoned the Iranian ambassador over the Fairford incident and that the government had imposed sanctions on two individuals accused of spying in Europe and the US.

“He [Miliband] will condemn Iran’s continued attempts to undermine our safety. We are also sanctioning two individuals involved in conducting espionage and attacks on behalf of the government of Iran in Europe and the US, and we will not hesitate to take further action to restrict Iran’s capacity to threaten international security,” she said.

Five men from London drove to Whelford, a village next to a base used by the US air force, in three white vans shortly after midnight on 27 September. They were released on police bail a day later after no bombs or explosives were found.

However, the incident was followed by the withdrawal of 12 B-1 bombers a week later, amid reports that the US was concerned about a possible drone attack on a base it had used to attack Iran during the Middle East war in the spring and summer.

Burnham’s brief remarks were a step up from previous comments. The prime minister had said in the days after the incident that there were “strong indications” Iran had played a part – but now appeared confident enough to apportion responsibility.

A joint statement from the UK, US and other European allies, released by the Foreign Office, added that the UK “has also concluded this event demonstrates the IRGC’s continued willingness to project hostile activity beyond its borders through covert networks and proxy actors”.

​However, despite his statement, it remains unclear what the authorities think happened at Fairford. Some experts have speculated it was a “feint” to demonstrate that Iran could in theory attack a relatively open site.

British sources have repeatedly maintained that Iran lacks the capability to launch what would be a sophisticated drone attack, but the incident appears to have unnerved the US, which is worried about the security of its bases in Europe.

The incident at Fairford comes after what the UK, in the joint statement, said was a recent campaign of attacks across Europe targeting Jewish communities, Iranian journalists and US interests, claimed by Harakat Ashab al-Yamin al-Islamiya (HAYI).

Though the US has detained and indicted Mohammad Baqer Saad Dawood al-Saadi, whom it accuses of being behind the HAYI group, there is evidence to suggest that Iran-linked attacks have continued.

A week ago, two Iranian nationals, Salam Ahmadyan from Liverpool and Rahman Salehi from Salford, were charged with terror offences, accused of trying to target the Jewish community in Manchester with a homemade chlorine powder bomb.

Prosecutors told a magistrates court the pair were in contact with a third party, believed to be in Iran, who directed them and sent them “instructions on how to prepare an explosive device”. A trial at the Old Bailey has been scheduled.