Andy Burnham has pointed the finger of blame at Tehran for the suspected terror plot at an RAF base.
The Prime Minister suggested that Iran was behind the incident, over which five Londoners have been arrested and bailed.
Donald Trump’s administration has said it believes a foreign actor was involved in the suspected plot that was ‘foiled’ at the weekend.
Asked by the BBC whether this was correct, Mr Burnham, speaking at Labour’s annual conference in Liverpool, said: “We believe so.
“There are strong indications that Iran played a part in what happened over the weekend at RAF Fairford, but of course it remains an ongoing, complex, and serious police investigation.
“Of course, we are working closely with our colleagues in the United States of America.”
He added: “So it's ongoing. I have held a Cobra meeting as I've been here at the Conference in Liverpool. We are on top of it in every possible way imaginable. Police in Gloucestershire, our counter-terrorism police, have done an outstanding job. I thank them for that.
“We will say more in due course, but we can confirm our belief that we believe Iran to have played a part.”
Asked about the bailing of the five Londoners, a move which has been criticised by Trump, the Prime Minister explained: “People need to understand that you know when it says released, that isn't as people might think. It's obviously with the strictest possible bail conditions.”
He added: “I have complete confidence in the way British security services are handling this.”
Five British nationals, all in their mid-20s, were arrested on suspicion of explosives offences and terrorism near the military base over the weekend before being bailed under investigation on Monday.
The Gloucestershire site has been used in recent months by American bombers after Sir Keir Starmer granted the US permission to use British bases for “defensive” strikes against Tehran during the Middle East war.
Counter-terror police have said a quantity of petrol but no explosive devices were found in the vans that sparked the security alert near the airbase.
Police cordons near the base are expected to be completely lifted on Wednesday, and searches of the London homes of the five suspects are complete.
Speculation over whether Iran had any involvement in the drama, including by tasking proxies to carry out crimes on their behalf on UK soil, remains rife.
Potential involvement by Iran was understood to be one line of inquiry for British investigators, and the nation has used proxies in the past to carry out acts in the UK.
But its London embassy fiercely denied any involvement in the events in Gloucestershire.
Speaking to Fox News, US Secretary of State Marco Rubio said: “It’s one that clearly involves the hand of a foreign actor. I won’t get into great detail about that yet.
“I do want to thank the authorities in the United Kingdom who have been very co-operative and on top of it.”
He added: “I know a lot of people are disturbed by the news that some of these people have been released on bail, as some of the people are being investigated, and obviously we’re engaging with them on all of that.
“Beyond that, I would argue that they’ve been very co-operative and very helpful on something that’s a very serious incident.”
Mr Trump said on Monday that the alleged plot “might be” linked to Iran, and added: “I will tell you, I would have said ‘Don’t let them have bail’. I’m surprised that they released them. I wouldn’t have done that.”
The five suspects, all UK nationals, are a 24-year-old from Camden, north London; and a 23-year-old, two 24-year-olds and a 25-year-old from Westminster, central London.
Police would not confirm when they are due to answer bail.
