Donald Trump threatened the UK with a 25 per cent tariff on its cars unless the British government officially accused Iran of breaking the 2015 nuclear deal, it has been reported.

The secret threat last week, first reported by The Washington Post, which cited unnamed European officials, would have seen the tariff imposed on all European automobile imports to the US unless Britain, France and Germany agreed to the ultimatum.

It came days before the three European Union powers on Tuesday triggered a dispute mechanism under the agreement which does amount to a formal accusation against Tehran of violating its terms. It could lead to the reinstatement of United Nations sanctions, but is being framed by the Europeans as the last chance to save the nuclear deal.

Iran said on Thursday the European countries had succumbed to a “high school bully”.

“Appeasement confirmed. E3 sold out remnants of #JCPOA to avoid new Trump tariffs,” Iranian foreign minister Mohammad Javad Zarif wrote on Twitter, using the acronym of the 2015 accord's official title.

Protests in Iran over downing of plane and tensions with the west Show all 23 1 /23 Protests in Iran over downing of plane and tensions with the west Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University Iranian police officers take position while protesters gather in front of Amir Kabir University AP Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University Iranians demonstrate following a tribute for the victims of the Ukraine Boeing 737 crash in front of the Amirkabir University in the capital Tehran EPA Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University Iranian police dispersed students chanting "radical" slogans during a gathering in Tehran to honour the 176 people killed when an airliner was mistakenly shot down AFP via Getty Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University - People hold flowers as tear gas fired by police rises Online videos purported to show that Iranian security forces fired both live ammunition and tear gas to disperse demonstrators protesting against the Islamic Republic's initial denial that it shot down a Ukrainian jetliner AP Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University Iranian police officers take position while protesters gather AP Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University AFP correspondents said hundreds of students had gathered early in the evening to pay respects to those killed in the air disaster ISNA/AFP via Getty Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University Iranian protester prepares to throw a tear gas canister back at police AP Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University A protester confronts an Iranian police officer AP Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University epa08120223 Iranians protest to show their sympathy with victims of Ukraine International Airlines Boeing 737-800 in front of the Amir Kabir University in Tehran, Iran, 11 January 2020. Media reported that hundreds of Iranians protests in Tehran in solidarity with victims of the Ukraine plane as Iranian military released a statement claiming that Ukraine International Airlines flight PS752 was shot down due to human error. EPA/ABEDIN TAHERKENAREH ABEDIN TAHERKENAREH EPA Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University Iranian police officers take position while protesters gathe AP Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University Iranians light candles for victims EPA Protests in Iran over downing of plane and tensions with the west Anti-government protest at Amirkabir University epa08120526 Iranians protest to show their sympathy to victims of Ukraine International Airlines Boeing 737-800 in front of the Amir Kabir University in Tehran, Iran, 11 January 2020. The Iranian military released a statement on 11 January 2020 that Ukraine International Airlines flight PS752 was shot down due to 'human error.' The passemger jet en route from Tehran to Kiev crashed minutes after takeoff on 08 January 2020, all all 167 passengers and nine crew members aboard were killed. EPA/ABEDIN TAHERKENAREH ABEDIN TAHERKENAREH EPA Protests in Iran over downing of plane and tensions with the west British embassy protest in Tehran People burn British and Israeli flags during a protest in front of the British embassy, in Tehran. Hundreds of Iranian hardliners gathered for a protest against Britain, a day after the UK ambassador to Iran Robert Macaire was detained after attending a a vigil for the victims of Ukraine passenger jet EPA Protests in Iran over downing of plane and tensions with the west British embassy protestBritish embassy protest in Tehran Chanting "Death to Britain", up to 200 protesters rallied outside the mission a day after the brief arrest of British ambassador Rob Macaire AFP via Getty Protests in Iran over downing of plane and tensions with the west British embassy protest in Tehran epa08122502 Iranian hardliners shout slogans as they gather in front of the British embassy during an anti-Britain protest, in Tehran, Iran, 12 January 2020. According to reports, hundreds of Iranian hardliners gathered for a protest against Britain, a day after the UK ambassador to Iran Robert Macaire was detained after attending a a vigil for the victims of Ukraine passenger jet. EPA/ABEDIN TAHERKENAREH ABEDIN TAHERKENAREH EPA Protests in Iran over downing of plane and tensions with the west British embassy protest in Tehran Iranian demonstrators hold placards bearing the images of slain military commander Qasem Soleimani and Iran's Supreme Leader Ayatollah Ali Khamenei in front of the British embassy in the capital Tehran on January 12, 2020 following the British ambassador's arrest for allegedly attending an illegal demonstration. - Chanting "Death to Britain", up to 200 protesters rallied outside the mission a day after the brief arrest of British ambassador Rob Macaire at a memorial for those killed when a Ukraine airliner was shot down. (Photo by ATTA KENARE / AFP) (Photo by ATTA KENARE/AFP via Getty Images) ATTA KENARE AFP via Getty Images Protests in Iran over downing of plane and tensions with the west Protesters chant slogans and hold up posters of Gen. Qassem Soleimani while burning representations of British and Israeli flags, during a demonstration in front of the British Embassy in Tehran, Iran, Sunday, Jan. 12, 2020. A candlelight ceremony late Saturday in Tehran turned into a protest, with hundreds of people chanting against the country's leaders â€” including Supreme Leader Ayatollah Ali Khamenei â€” and police dispersing them with tear gas. Police briefly detained the British ambassador to Iran, Rob Macaire, who said he went to the Saturday vigil without knowing it would turn into a protest. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP Protests in Iran over downing of plane and tensions with the west British embassy protest in Tehran epa08122741 Iranian hardliners gather in front of the British embassy during an anti-Britain protest, in Tehran, Iran, 12 January 2020. According to reports, hundreds of Iranian hardliners gathered for a protest against Britain, a day after the UK ambassador to Iran Robert Macaire was detained after attending a a vigil for the victims of Ukraine passenger jet. EPA/ABEDIN TAHERKENAREH ABEDIN TAHERKENAREH EPA Protests in Iran over downing of plane and tensions with the west A hardline protester holds up a placard during a demonstration in front of the British Embassy in Tehran, Iran, Sunday, Jan. 12, 2020. A candlelight ceremony late Saturday in Tehran turned into a protest, with hundreds of people chanting against the country's leaders â€” including Supreme Leader Ayatollah Ali Khamenei â€” and police dispersing them with tear gas. Police briefly detained the British ambassador to Iran, Rob Macaire, who said he went to the Saturday vigil without knowing it would turn into a protest. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP Protests in Iran over downing of plane and tensions with the west British embassy protest in Tehran epa08122743 Iranian hardliners gather in front of the British embassy during an anti-Britain protest, in Tehran, Iran, 12 January 2020. According to reports, hundreds of Iranian hardliners gathered for a protest against Britain, a day after the UK ambassador to Iran Robert Macaire was detained after attending a a vigil for the victims of Ukraine passenger jet. EPA/ABEDIN TAHERKENAREH ABEDIN TAHERKENAREH EPA Protests in Iran over downing of plane and tensions with the west British embassy protest in Tehran epa08122745 Iranian hardliners gather in front of the British embassy during an anti-Britain protest, in Tehran, Iran, 12 January 2020. According to reports, hundreds of Iranian hardliners gathered for a protest against Britain, a day after the UK ambassador to Iran Robert Macaire was detained after attending a a vigil for the victims of Ukraine passenger jet. EPA/ABEDIN TAHERKENAREH ABEDIN TAHERKENAREH EPA Protests in Iran over downing of plane and tensions with the west British embassy protest in Tehran epa08122786 Iranian hardliners gather in front of the British embassy during an anti-Britain protest, in Tehran, Iran, 12 January 2020. According to reports, hundreds of Iranian hardliners gathered for a protest against Britain, a day after the UK ambassador to Iran Robert Macaire was detained after attending a a vigil for the victims of Ukraine passenger jet. EPA/ABEDIN TAHERKENAREH ABEDIN TAHERKENAREH EPA Protests in Iran over downing of plane and tensions with the west Protesters chant slogans while holding up posters of Gen. Qassem Soleimani during a demonstration in front of the British Embassy in Tehran, Iran, Sunday, Jan. 12, 2020. AP

“It won’t work my friends. You only whet his appetite. Remember your high school bully?”

The 2015 pact was agreed between Tehran and world powers, offering Iran relief from sanctions if it curbed its nuclear work. Mr Trump withdrew from the deal in 2018 and reimposed US sanctions, telling Tehran he wanted a more stringent deal on nuclear and other issues.

Iran has responded to the US sanctions by scaling back its compliance with the deal, culminating with an announcement this month that it would reject all limits on production of enriched uranium, although it says it wants to keep the deal in place.

Boris Johnson said a “Trump deal” could replace the existing one, were it to fail, while Paris said broad talks were needed.

Two European diplomats confirmed to the Washinton Post that the US had threatened tariffs but said leaders of the three European states had already decided to trigger the mechanism before that.

A third diplomat said such US tactics only undermined the Europeans, who are trying to apply pressure independently.

“True or not it has the effect of discrediting the Europeans, but then Trump doesn’t really care about that,” the diplomat said. “From the Iranian side, it just proves that only the US matters in this.”

The Europeans have long opposed Mr Trump’s decision to withdraw from the nuclear deal, but have been threatening for months to trigger the dispute mechanism if Iran did not comply with it.

Effigy of UK ambassador to Iran set on fire in Tehran

The dispute mechanism begins a complex diplomatic process that can end with UN sanctions on Iran “snapping back” into place, although the Europeans say that is not their aim.

In triggering the dispute mechanism, the European countries said they were not backing a US policy of “maximum pressure” on Iran, and they still hope to salvage the nuclear deal.

The nuclear dispute lies at the heart of Iran’s long-running standoff with the West that spiralled into open conflict this month when Washington killed Iranian general Qassem Soleimani in Baghdad and Tehran responded with missile strikes on US targets in Iraq.

During that period of high alert, Iran shot down a civilian airliner in what it says was a tragic mistake. It has triggered anti-government protests at home.

Enriched uranium can be used to create material for nuclear warheads. Iran denies Western accusations it wants such weapons and says it wants nuclear material for peaceful purposes.

“We are enriching more uranium than before the deal was reached,” president Hassan Rouhani said in a televised speech. “Pressure has increased on Iran but we continue to progress.”

US sanctions meanwhile have hammered Iran’s economy. Washington aims to reduce its oil exports to zero.