Daniel Stenberg, who is also the author of the command-line tool curl, was told at the airport in Sweden that he could not check in for his flight because the ESTA (electronic system for travel authorisation) waiver he had obtained had been turned down.

It is not known if his being turned away has anything to do with the travel bans that were put in place by the Trump administration. After courts prevented the bans from being enforced, the Supreme Court recently allowed the ban to stand for citizens of six nations who had no connection to the US and also for refugees. The court is due to hear a challenge to the ban later in the year.

Gizmodo quoted Stenberg as saying: “The lady (at the airport check-in counter) saw something on her screen when she entered info about me."

A number of countries. including Australia, have a visa waiver arrangement with the US whereby a citizen of these countries can obtain a permission slip online which allows them to enter the US for a period not exceeding 90 days. Businessmen and journalists are not allowed to use this facility when they travel to the US for work.

The Mozilla Foundation has opposed the travel ban from the start. Initially, seven countries — Iran, Iraq, Yemen, Syria, Sudan, Libya and Somalia — were covered, but it now appears that Iraq has been removed from the list.

Mozilla’s chief business and legal officer Denelle Dixon told Gizmodo: “We are focused on providing support for our employees and we’re looking into ways to resolve and understand the circumstances. Currently, we do not believe the situation is related to the US Administration’s proposed travel ban or the recent Supreme Court ruling."

Stenberg said he could not think of any reason why he would not be allowed to enter the US. He was worried that he had been blocked from entering, and feared that it would come to haunt him in the future too.

A Customs and Border Protection spokesperson said that 1.2 million people entered the US every day and about 700 were turned back. "Having an approved ESTA does [not] guarantee a foreign national free entry into the US All travellers including those coming from visa waiver countries must clear all 60 grounds of inadmissibility," the spokesperson said.