The World Health Organisation said the Indian variant of the coronavirus is "poised to take hold" of Europe as many countries prepare to ease restrictions and cross-border travel resumes.
Hans Kluge, WHO’s Europe director, said the continent was “by no means out of danger” despite a steady decline in infection rates in recent weeks.
He said the Indian variant, also known as the Delta variant, was showing signs of evading some vaccines and that many vulnerable people, particularly those over 60, remained unprotected.
“The new Delta variant of concern … is poised to take hold in the region,” he said.
“We have been here before. Over the course of last summer, cases gradually rose in younger age groups and then moved into older age groups, contributing to a devastating resurgence.”
In the UK, Health Secretary Matt Hancock said the Indian variant now accounted for 91 per cent of all new cases in the country.
The spread of the strain forced ministers to reconsider plans for the final stage of unlocking on June 21.
Prime Minister Boris Johnson is due to scrutinise data before announcing on Monday whether he will delay that last step.
Dr Kluge urged travellers to use common sense ahead of the peak summer season.
"With increasing social gatherings, greater population mobility and large festivals and sports tournaments taking place in the coming days and weeks, WHO Europe calls for caution," he said.
"If you choose to travel, do it responsibly. Be conscious of the risks. Apply common sense and don’t jeopardise hard-earned gains.”
Over the past two months, new coronavirus cases, deaths and hospital admissions have declined in Europe, prompting 36 out of 53 countries in the continent to start easing social restrictions.
The number of reported Covid-19 infections in Europe last week was 368,000, a fifth of the weekly cases reported during a peak in April this year.
About 30 per cent of Europe’s population has received a first dose of a Covid-19 vaccine.