Boris Johnson said General Qassem Soleimani was a 'threat to all our interests' as he called for a de-escalation of violence following his death.
The Prime Minister issued his first statement on the crisis after the US carried out a fatal drone strike, killing the military leader.
Mr Johnson said any acts of retaliation were in 'no one's interest' and would lead to more violence.
General Soleimani was assassinated in Baghdad on Friday.
The Prime Minister, who had faced criticism for his silence while on holiday in the Caribbean, said: "General Qassem Soleimani posed a threat to all our interests and was responsible for a pattern of disruptive, destabilising behaviour in the region.
"Given the leading role he has played in actions that have led to the deaths of thousands of innocent civilians and western personnel, we will not lament his death.
"It is clear however that all calls for retaliation or reprisals will simply lead to more violence in the region and they are in no one's interest."
Mr Johnson has spoken to US President Donald Trump, as well as President Emmanuel Macron and German Chancellor Angela Merkel, Downing Street confirmed.
The PM said he planned to speak to leaders in Iraq to support 'peace and stability'.
Its parliament called for the expulsion of foreign troops, including British soldiers working against so-called Islamic State.
Mr Johnson arrived back in the UK on Sunday, Downing Street confirmed.
Ministers are due to meet on Monday to discuss the crisis that has raised fears of all-out war.
The Foreign Office will update Parliament on Tuesday.
Around 400 UK troops are stationed in Iraq, while the US has 5,200. It has prompted fears of a withdrawal that could cripple the battle against the terror group IS.
The Ministry of Defence was understood to be awaiting the decision of the Iraqi government before acting over the soldiers based there as part of the US-led coalition.
A UK Government spokesman said: "The coalition is in Iraq to help protect Iraqis and others from the threat from Daesh (Islamic State), at the request of the Iraqi government.
"We urge the Iraqi government to ensure the coalition is able to continue our vital work countering this shared threat."
In response to the killing of Gen Soleimani on Friday, Iraqi prime minister Adel Abdul-Mahdi said the parliament could end the presence of foreign troops or restrict their mission training local forces. He backed the first option.
Foreign Secretary Dominic Raab had spoken to him on Sunday morning in the wake of the killing of the head of the elite Quds Force, who masterminded Tehran's security strategy in the region.
Mr Raab was defending Mr Trump's decision to launch the drone strike, accusing hardliners in Tehran of "nefarious behaviour" and saying the US has the "right of self defence".
But his call for the pursuit of a diplomatic route to bring Tehran "in from the international cold" came as Iran accused the US president of breaching international law.
"The US will take their own operational judgment call but they've got the right of self defence," Mr Raab told Sky's Sophy Ridge on Sunday.
"So we understand the position the US were in and I don't think we should be naive about the Iranian Revolutionary Guard or indeed General Soleimani."
The Foreign Secretary also defended Mr Johnson, saying he has been in "constant contact" with the PM who remained "in charge" throughout his holiday.
As Mr Raab was speaking, Iran's foreign minister Mohammad Javad Zarif struck back at the President's Twitter threat to target 52 Iranian sites "very fast and very hard" if Tehran attacks US assets.
Mr Zarif accused Mr Trump of having "committed grave breaches" of international law with the killing and of threatening to commit a "war crime" by targeting cultural sites.
Labour's shadow foreign secretary Emily Thornberry warned of a "lurch towards war" arising from the president's "reckless" decision to kill the general.
The Foreign Office strengthened travel advice to Britons across the Middle East including Saudi Arabia and Turkey, while the Navy was to begin accompanying UK-flagged ships through the key oil route of the Strait of Hormuz.
