BBC:
Dominick Chilcott told the BBC Iran is a country in which such action is only taken "with the acquiescence and the support of the state".
Hundreds of protesters attacked the UK embassy in Tehran on Tuesday.
And diplomats working at the Iranian embassy in London have left Britain after being expelled.
The Iranian diplomats expelled from the UK flew out of Heathrow on a chartered Iran Air flight on Friday afternoon.
Their expulsion was ordered by Foreign Secretary William Hague after the British embassy in Tehran was stormed on Tuesday.
Iran said it regretted the incident, which it described as "unacceptable behaviour by a small number of protesters".
But Mr Chilcott, the newly appointed British ambassador to Iran, told the BBC's Gavin Esler the attack was likely to have had state backing.
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He said: "Iran is not the sort of country where spontaneously a demonstration congregates and then attacks a foreign embassy. That sort of activity is only done with the acquiescence and the support of the state.
"And there are a number of reasons why, with the benefit of hindsight, it's very clear that this was a state-supported activity."
He also said some within Iran's ruling regime may have underestimated the British response.
"The risk is that certain people in the regime who liked the idea of confrontation, because they felt it would rally people to the flag, miscalculated how strong the response would be," said Mr Chilcott.
"They probably didn't expect us to send home the Iranian embassy in London and, reading between the lines, you can see in the way they have responded to that move, some remorse in having provoked it. I think that might apply more generally too," he added.
Earlier, Iranian diplomatic staff boarded a plane at Heathrow Airport, London, heading to Tehran
Meanwhile, Deputy Prime Minister Nick Clegg told the BBC's Sarah Rainsford the UK's relationship with Iran had "taken a very serious knock".
He said: "Something really bad happened when the Iranian authorities allowed those people to overrun our embassy compounds, and it is quite right that we have been very clear in our response - as have many other European countries who have withdrawn their ambassadors for consultations.
"It doesn't mean we're cutting off all diplomatic relations with Iran. It doesn't mean we are in any way lessening our determination to try to find a diplomatic solution to the nuclear question, which is immensely important to Europe and the whole world, and we will continue to work tirelessly to find a negotiated solution."
On Wednesday, Mr Hague told MPs: "If any country makes it impossible for us to operate on their soil, they cannot expect to have a functioning embassy here."
The foreign secretary said there had been "some degree of regime consent" in the attacks on the embassy and on another UK diplomatic compound in Tehran.
He also said all UK diplomatic staff in Tehran had been evacuated and the embassy closed.
