Travel corridors, which exempted people from needing to self-isolate when returning, were linked to higher rates of onward transition of imported cases, the researchers said.
They argued the genomics study demonstrates the effectiveness of travel restrictions in reducing the spread of Covid-19, with restrictions linked to a 40 per cent lower rate of contacts.
The mean number of cases of a specific genetic variant detected four weeks after likely import was 11.3 when travel restrictions were not in place, compared to 1.2 when they were.
Travel from the European countries accounted for 86 per cent of imported cases between May and September, according to the study, which is yet to be peer-reviewed.
Greece, which had quarantine-less travel for the entire period, was “the source of greatest imported” Covid-19 cases, accounting for 21 per cent of travel-linked cases, or 882 infections in the study of 4,207 cases.
The next highest were Croatia, which was taken off the exemption list on August 22 and accounted for 16 per cent, and Spain, removed on July 26 and accounting for 14 per cent.
“This highlights the need for active surveillance of imported cases of Sars-CoV-2 for the introduction of travel corridors in a timely manner,” the study, worked on by academics from PHE, Cambridge University and the Covid-19 Genomics UK consortium, said.
The travel corridor policy introduced in July exempted people from 14 days of quarantine when returning from countries deemed safe, a period later reduced to 10.
A Department for Transport spokesman said: “As this report rightly points out, travel restrictions imposed by Government were effective in reducing the transmission of imported cases of Covid-19.
“Guided by the latest scientific data, the Government acted rapidly to remove travel corridors with Greece and Spain, and we continue to have robust measures in place to protect the country against imported cases of the virus.”
Harry Theoharis stressed Athens was ready to reach such an agreement even if it meant doing so unilaterally rather than as part of a broader deal with the EU.
“When you travel to Greece, you do not need to pack your smile but you are guaranteed that on your way back you will have your smile,” he told ITV’s Good Morning Britain.
Asked whether he was confident of reaching arrangements for Britons to easily travel to Greece, he responded: “Yes, of course we will and we will do everything in our power even unilaterally to make things easier for our friends from Britain, and in many cases come to Greece every year.
“We are in discussions with the British government to ensure that this is also done in a way that is safe that keeps us away from fake certificates or anything like that.”
He said that Greece wanted a vaccine passport system, digital or possibly even paper as well, which would allow people entry if they had had two vaccine doses, a negative result or proof that they have recovered from the disease such as antibodies.
Additional reporting from PA Media