The United Kingdom reported 45,140 new cases of COVID-19 on Sunday.
That's the highest daily number since mid-July.
Former FDA Commissioner Scott Gottlieb tweeted: "U.K. reported its biggest one-day COVID case increase in three months — just as the new Delta variant with the mutation in the spike reaches 8% of U.K. sequenced cases. We need urgent research to figure out if this Delta-plus is more transmissible."
Britain's surging COVID numbers has it falling behind the rest of Europe in recovery from the pandemic.
The country was faster to relax its restrictions and reopen than other European countries.