Data from the Institute of Records and Notary show that last year alone 56 citizens of these countries - 32 from Iran, 12 from Iraq, one from Libya, six from Somalia, three from Syria and two from Yemen - requested Portuguese citizenship. Of those, 41 were under the law that allows for the attribution of nationality to foreigners resident in Portugal.

The recently inaugurated administration of President Donald Trump last week decided to bar the entry during 90 days to people from these Muslim-majority countries.

Over the last three years 75 out of the total of 108 applications for Portuguese nationality were on the basis that the applicant was already resident. Other reasons given for seeking Portuguese nationality were the fact of their being minors born on Portuguese territory (in the case of 12 of them), of being married or living with a Portuguese citizen (11 of them), of being the children of foreigners but born in Portugal (five of them), of being born abroad but to Portuguese citizens (one case), or by statement of desire (four cases).

In 2015 the total number of applicants was 29, and in 2014 there were 23.

The Trump administration's move has prompted criticism from various quarters, including the new secretary general of the United Nations, António Guterres, a former prime minister of Portugal. He said that the ban violates “basic principles” and should be revoked.

He also argued that the measures “are not effective if the objective is really to avoid the entry of terrorists”.

There have been significant demonstrations at airports in the US and on the streets of various European countries to protest against the ban.

Some airlines have refused to allow passengers to embark on flights to the US if there are fears that they may not be able to enter the country as a result of the ban. Portugal's national carrier, TAP, announced on Wednesday that clients with tickets to the US, or which entail a stopover there, and who are covered by the ban, may change them to another date or destination or request a refund.

On Monday Portugal's Foreigners and Borders Service (SEF) told Lusa that it “has not altered any kind of procedure and continues to enforce national and European laws, as is its duty."