Questions are being asked about outside influence on Donald Trump after the US president called off air strikes against Iran at the last minute.

Having initially said he believed Iran made an error when it shot down a US drone on Thursday in the Strait of Hormuz, the president nonetheless reportedly approved military strikes against the Islamic Republic later that day.

The operation to hit targets such as radars and missile batteries was in its initial stages, the New York Times reported, and planes were in the air and ships had been moved into position. But before any missiles were fired the operation was cancelled.

It was not clear if Mr Trump had changed his mind, or whether the strike was called off for operational or strategic reasons, said the report, but Dr Karen von Hippel, a former senior adviser at the State Department said she believed “somebody obviously got to” the president.

“It’s really hard to say with President Trump, and I certainly wouldn’t consider myself to be a Trump whisperer, but somebody obviously got to him, and whether that was Tucker Carlson from Fox News or prime minister Trudeau, it’s hard to say, “ Dr Von Hippel, now director-general of the Royal United Services Institute, told BBC Radio 4’s Today programme.

Tensions high as Trump approves new Iran sanctions: In pictures Show all 17 1 /17 Tensions high as Trump approves new Iran sanctions: In pictures Tensions high as Trump approves new Iran sanctions: In pictures Iran claims that in this picture released by Iran state TV, their surface-to-air missile is seen as it shoots down a US surveillance drone EPA Tensions high as Trump approves new Iran sanctions: In pictures This photo shows US RQ-4A Global Hawk unmanned surveillance drone. A drone of this model was shot down by Iran on Thursday 21 June AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures An oil tanker is on fire after it was subject to a suspected attacked at the Gulf of Oman on June 13. The US has blamed Iran for the attack Reuters Tensions high as Trump approves new Iran sanctions: In pictures Iran claims that in this picture released by Iran state TV, debris from the downed US drone is seen after it was recovered from Iranian waters AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures Smoke billows from an oil tanker after it was subject to a suspected attacked at the Gulf of Oman on June 13 Reuters Tensions high as Trump approves new Iran sanctions: In pictures US President Trump holds up a signed executive order to increase sanctions on Iran on 24 June AP Tensions high as Trump approves new Iran sanctions: In pictures Iranian President Rouhani stated in a televised address that the White House is "afflicted by mental retardation" following the increase in sanctions on 25 June EPA Tensions high as Trump approves new Iran sanctions: In pictures An Iranian navy boat tackles the fire on the Norwegian owned Front Altair oil tanker after it was hit in a suspected attack AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures Iran claims that in this picture released by Iran state TV, debris from the downed US drone is seen after it was recovered from Iranian waters AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures Smoke billows from an oil tanker after it was subject to a suspected attacked at the Gulf of Oman on June 13 Reuters Tensions high as Trump approves new Iran sanctions: In pictures A screenshot from a video released by the US Department of Defense that the US claims to show Iranian removing an unexploded limpet mine form the hull of the Japan-owned ship that was attacked in the Gulf of Oman on June 13 Getty Tensions high as Trump approves new Iran sanctions: In pictures An item reportedly found on the Japan-owned oil tanker that was attacked on June 13 in the Gulf of Oman AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures A handout photograph from the US Department of Defense shows a composite material that the US claim was left behind on the hull of the Japan-owned oil tanker following the removal of an unexploded limpet mine Getty Tensions high as Trump approves new Iran sanctions: In pictures A view from the cabin of an Iranian navy boat as it tackles the fire on the Norwegian owned Front Altair oil tanker after it was hit in a suspected attack EPA Tensions high as Trump approves new Iran sanctions: In pictures Release by US government A picture released by U.S. Central Command shows damage to the hull of the oil tanker Kokuka Courageous. The picture suggests that the ship is 'likely' to have been hit by a mine as the US Secretary of State Mike Pompeo moves to blame Iran for the suspected attack Reuters Tensions high as Trump approves new Iran sanctions: In pictures Smoke billows from an oil tanker after it was subject to a suspected attacked at the Gulf of Oman on June 13 Reuters Tensions high as Trump approves new Iran sanctions: In pictures Release by US government A picture released by U.S. Central Command shows damage to the hull of the oil tanker Kokuka Courageous. The pictures suggests that the ship is 'likely' to have been hit by a mine as the US Secretary of State Mike Pompeo moves to blame Iran for the suspected attack EPA

Mr Trump had met with Canadian leader Justin Trudeau earlier that day, while the Daily Beast reported Mr Carlson has privately advised the president against military action in Iran.

It comes in contrast to the hawkish position on Tehran taken by two of the most senior members of the Trump administration: secretary of state Mike Pompeo and particularly John Bolton, Mr Trump’s national security adviser.

CNN reported Mr Bolton, who has repeatedly called for Iranian regime change in the past, has been locked in “debate” with Mr Trump over how to handle Iran, with other senior officials playing the role of “swing votes”.

On Fox News, there appears to be a battle for Mr Trump’s ear after Sean Hannity, another host and longtime friend of the president, on Thursday night called on the US to “bomb the hell out of Iran”.

“A strong message needs to be sent that a huge price will be paid if you take on the United States of America,” Mr Hannity said. “Simple peace through strength, and it works.”

Mr Carlson on Monday devoted a segment of his show to question the shaky evidence provided by US authorities purporting to prove Iran was responsible for an attack on two commercial oil tankers in the Gulf of Oman.

He attacked Mr Pompeo's "misplaced certainty" over Iran's responsibility and compared it to the Bush administration's discredited justifications of going to war with Iraq in 2003.

“We’re still paying a price for that,” Mr Carlson said.

Dr Von Hippel speculated that Mr Trump may have followed Mr Carlson’s advice.

“He’s been anti-war in the Middle East, and he’s been pushing Trump very hard not to do anything with Iran, and even challenged Pompeo’s intelligence briefing the other day on Fox News, and we know Trump really likes him and listens to him,” she said of Mr Carlson.

Iran military releases footage of 'missile strike on US drone'

She added: “With Trump it seems that he has one approach and it’s the same approach in business as foreign policy, and he likes to push very hard to the brink, and he assumes others will cave, and in foreign policy it just doesn’t work that way.”

Ben Rhodes, former national security adviser to president Barack Obama, said the confusion surrounding strikes against Iran demonstrated an “absence of any rational, coherent process for national security decision making” that “has always been a clear risk under Trump”.

“Now we see what that looks like in a crisis,” he added.

Former US ambassador to Russia, Michael McFaul, suggested the leak over Mr Trump’s U-turn could itself be an attempt by a Trump administration official to force the president’s hand into striking Iran by making him “look weak”.

Iranian officials told Reuters on Friday that Tehran had received a message from Mr Trump through Oman overnight warning a US attack on Iran was imminent.

“In his message, Trump said he was against any war with Iran and wanted to talk to Tehran about various issues ... He gave a short period of time to get our response but Iran’s immediate response was that it is up to Supreme Leader (Ayatollah Ali) Khamenei to decide about this issue,” one of the officials told Reuters, speaking on condition of anonymity.

The circumstances of the shooting down of the drone, a US navy RQ-4A Global Hawk, by Iran’s Revolutionary Guard, are disputed.

Iran said the drone, with a wingspan larger than a Boeing 737 and costing more than over $100m (£79m), had violated its territorial airspace.

The US said the "unprovoked attack” happened in international airspace, but failed to provide compelling evidence for the claim. Either way, it marked the first time Iran had struck the US military, which released a set of coordinates it claimed the drone was shot down at.

Iranian foreign minister Mohammad Javad Zarif said on Twitter that the aircraft had taken off from the United Arab Emirates “in stealth mode & violated Iranian airspace”.

A Revolutionary Guards statement said the drone’s identification transponder had been switched off “in violation of aviation rules and was moving in full secrecy” when it was downed, Reuters quoted the Iranian state broadcaster IRIB as saying.