Open this photo in gallery Former prime minister Stephen Harper speaks in Washington on March 26, 2017. Jose Luis Magana/The Canadian Press

Stephen Harper joined other former world leaders in writing a full-page endorsement in Thursday’s New York Times of U.S. President Donald Trump’s withdrawal from the 2015 Iran nuclear deal and his reinstatement of U.S. sanctions.

Mr. Harper and his former foreign affairs minister, John Baird, as well as former Australian prime minister John Howard and David Trimble, the former first minister of Northern Ireland, were among the 12 signatories, who said they “stand in complete support” of Mr. Trump’s hard-line stance on Iran.

“Iran is a danger to us, to our allies, to freedom,” they wrote. “An Islamist and revolutionary regime, such as the one that controls Tehran today, must never be allowed to possess a nuclear option, not a bomb, not a path to a bomb, not a nuclear program with the slightest doubt of its extent and military applications.”

Story continues below advertisement

The statement, paid for by Rafael L. Bardaji, the director of Miami-based consultancy World Wide Strategy, echoed the assertion of Israeli Prime Minister Benjamin Netanyahu that Iran has not honoured the international agreement. Iran has kept its nuclear infrastructure and carried out advanced research and development while fomenting terrorism in the region, Mr. Harper and his fellow signatories wrote.

“None of the promises that supporters of the deal made have proven correct,” the signatories said. “Iran’s regime has used this time to upgrade its long-range ballistic missile program and expand its violence against the people of Yemen, Syria, Iran and Lebanon while heading a list of state sponsors of terrorism.”

Prime Minister Justin Trudeau said Thursday that Canadian foreign policy is only made in Canada. Trudeau commented after former prime minister Stephen Harper signed a newspaper ad supporting the U.S. decision to withdraw from the Iran nuclear deal. The Canadian Press

The endorsement by the former Conservative prime minister, whose government severed diplomatic relations with Iran in 2012 and who was a staunch supporter of Mr. Netanyahu, stands in stark contrast to the position taken by Prime Minister Justin Trudeau. On Thursday, Mr. Trudeau said he continues to support the agreement, which puts limits on Iran’s ability to develop nuclear weapons in exchange for some sanctions relief.

“Mr. Harper is a private citizen and allowed his own opinions,″ Mr. Trudeau said in Saguenay, Que. “As for the government of Canada, we take a firm position that the [Iran deal], while not a perfect accord, certainly is a very positive step, holding Iran to account and preventing Iran from developing nuclear weapons, which would be a threat not just to the region but to global stability.″

He also wondered if the Conservative Party agrees with its former leader.

Conservative foreign affairs critic Erin O’Toole said Mr. Harper no longer sets policy, but he criticized Mr. Trudeau for bemoaning the U.S. decision to pull out of the deal.

“It is always best to have an agreement in place that is verifiable, and we understand President Trump’s concerns and that is why we would rather push for a new agreement than just regretting the fact that the U.S. has walked out of it,” Mr. O’Toole said in an interview. “Let’s get the President’s concerns addressed in a new deal.”

Story continues below advertisement

A Canadian official said Ottawa is not planning to reimpose sanctions on Iran or restrict any Canadian companies that want to do business there. The official also pointed out that every other country that agreed to the deal – Germany, the United Kingdom, France, China and Russia – is sticking to it, as is Canada.

The official added, however, that Canadian firms looking to invest in Iran would find it difficult, given that so much of the international financial system is tied to the United States and any investments would have to comply with U.S. sanctions.

Foreign Affairs Minister Chrystia Freeland said she had spoken with her British counterpart, Boris Johnson, about how the Iran deal could work in the wake of Mr. Trump’s withdrawal. Iran has suggested it may demand further reassurances for holding up its end of the deal.

“We talked about a path forward and ways the global community can continue to work together to prevent Iran from obtaining nuclear weapons,” Ms. Freeland told reporters outside the U.S. Capitol in Washington, where she is renegotiating NAFTA this week.

The U.S. decision is expected to be a topic of discussion when Mr. Trudeau hosts Mr. Trump and their G7 counterparts in Quebec at their annual summit in June.

Israel’s envoy to Canada, Nimrod Barkan, said he was “happy” that Mr. Harper and other former world leaders, including the former foreign ministers of the Netherlands and Italy, are backing the “wise decision” of Mr. Trump.

Story continues below advertisement

“Trump’s decision actually gives the Western countries the leverage they were lacking to curb Iran’s development of long-range missiles and its deployment of rockets to Syria that continue to threaten Israel,” he said in an interview Thursday.