Donald Trump’s executive order barring immigration from seven Muslim-majority countries is illegal, according to a policy expert.

The US president suspended refugee admissions to the country for 120 days under measures to “keep radical Islamic terrorists out.”

However immigration policy analyst David Bier argued the order was unconstitutional due to a law implemented by Congress more than 50 years ago.

The expert from the Cato Institute’s Center for Global Liberty and Prosperity cited the Immigration and Nationality Act of 1965 in an op-ed piece for the New York Times, which forbids all discrimination against immigrants on the basis of national origin.

Mr Trump’s order, titled 'Protecting the Nation From Foreign Terrorist Entry Into The United States', banned Syrian refugees indefinitely until “significant changes” are made, and put a temporary block on visitors from “countries of particular concern”, including Syria, Iraq, Iran, Libya, Somalia, Sudan and Yemen.

“Mr. Trump asserts that he still has the power to discriminate, pointing to a 1952 law that allows the president the ability to 'suspend the entry' of 'any class of aliens' that he finds are detrimental to the interest of the United States,” he wrote in the New York Times. “But the president ignores the fact that Congress then restricted this power in 1965, stating plainly that no person could be 'discriminated against in the issuance of an immigrant visa because of the person’s race, sex, nationality, place of birth or place of residence'."

He also said the 1965 law was designed to give Americans the freedom to sponsor family members or marry a foreign-born spouse.

“If Mr Trump can legally ban an entire region of the world, he would render Congress’s vision of unbiased legal immigration a dead letter,” he wrote.

“An appeals court stopped President Barack Obama’s executive actions to spare millions of undocumented immigrants from deportations for the similar reason that he was circumventing Congress. Some discretion? Sure. Discretion to rewrite the law? Not in America’s constitutional system.”

It followed reports that green card and visa holders were being blocked from boarding US-bound flights within hours of Mr Trump issuing the order.

US airports were also said to have ordered some passengers who had managed to board flights to return to their point of origin, according to the Arab-American Anti-Discrimination Committee (ADC).

Dutch airline KLM confirmed it had refused carriage to seven passengers on Saturday.

"Worldwide, we had seven passengers whom we had to inform that there was no point in us taking them to the US," spokesperson Manel Vrijenhoek said.