The rapid spread of the Alpha coronavirus was caused by mutations and was helped by large numbers of infected people "exporting" it to parts of the UK, in what a new study calls a "super-seeding" event.
Results in the journal Science map the spread of the variant from its origins in Kent and Greater London in November 2020 to all but five counties in Wales, Scotland, Northern Ireland and England by January 19.
"At the beginning of December 2020, the [centre] of Covid-19 transmission in England shifted rapidly from the north-west and north-east, to London and the south-east, as the Alpha variant took hold," said Dr Moritz Kraemer, lead author on the study.
"As people travelled from London and the south-east to other areas of the UK, they ‘seeded’ new transmission chains of the variant.
"This continued as a national ‘super-seeding’ event, which did not start to slow until early January.
"Although travel was curbed after restrictions were introduced on December 20, this was compensated for by the continued exponential growth in Alpha variant cases."
The rapid spread of the Alpha strain across the UK led to initial reports that it could be up to 80 per cent more transmissible than the original strain.
Although it is not specifically blamed by the researchers, the period covers the build up to Christmas when UK Prime Minister Boris Johnson at first promised festivities should be able to go ahead before clamping down as cases spiralled. Despite the plea, many people still returned to families around the country.
This study, published on Thursday by researchers at universities including Oxford, Northeastern and Edinburgh, shows travel significantly affected its spread and early growth rates.
The researchers say this highlights the need for epidemiologists to work closely and quickly with virologists and geneticists for more accurate transmissibility estimates for new variants.
"The Alpha variant does contain genetic changes that make it more transmissible," said Prof Oliver Pybus, lead researcher of the Oxford Martin Programme on Pandemic Genomics.
"It is likely the Alpha variant was 30 per cent to 40 per cent more transmissible than the initial strain.
"And the early estimates were higher because we did not know how much its growth was exacerbated by human mobility and by how many contacts different groups of people have.
"Crucially, as more variants emerge and spread in other countries worldwide, we must be careful to account for these phenomena when evaluating the intrinsic transmissibility of new variants."
Verity Hill, co-author and researcher from the University of Edinburgh, said the Alpha variant began by spreading mostly within London and the south-east, even during the November lockdown in England.
When the lockdown was lifted, the strain spread rapidly across the country, as human movement increased significantly, Dr Hill said.
Dr Kraemer said: "As new variants emerge, we expect they will spread significantly before travel restrictions are put in place, as probably happened with the Delta variant.
"Given the scale of its current outbreak, it seems probably that the UK is now an important exporter of the Delta variant across Europe and some other parts of the world.
"The UK has decided to ease its restrictions because of our high vaccination rates and a confidence that we have protected the most vulnerable people in society.
"But that is not the case in most other countries and the Delta variant could be starting this process again elsewhere, highlighting the urgent need for faster and equitable distribution of vaccines worldwide."