At least one Premier League club has suffered a coronavirus outbreak among its squad following the return of its players for pre-season training, Telegraph Sport can reveal.

As scientists warn teams to plan for a surge in positive cases as they reconvene following their summer holidays, at least one manager fears the resulting disruption will be the worst of the Covid-19 crisis.

In the past fortnight  top-flight players who are not involved in the European Championship or other international fixtures have returned from their longest break away from the game since the pandemic began.

Their time spent outside the squad environment – in which they were subject to strict protocols to limit the spread of the virus – has coincided with a surge in new cases across the country, particularly among men under the age of 30.

The Telegraph has been told that two players at one club have fallen ill with the virus, with a staff member also having to quarantine after his children tested positive.

Some lower-league teams are also reported to have been travelling around the UK for pre-season training camps on public transport rather than on Covid-secure coaches.

Monday’s announcement by Boris Johnson that almost all coronavirus restrictions would end on July 19 did not include scrapping the requirement for those with the virus – or their close contacts – to self-isolate.

That means an outbreak still has the ability to wreak havoc with elite sport, which was planning to retain some of the protocols developed for its original return.

Professor John Ashton, the former director of public for north-west England, told Telegraph Sport: “All bets are off, really. We seem to have thrown all caution to the wind on July 19.

“The number of new cases – predominantly of the Delta variant – are escalating very fast, especially among 15-29-year-olds, which is going to include quite a lot of footballers and people they associate with.”

Prof Ashton warned the virus was most likely to spread in indoor hospitality settings – “the sort of places footballers tend to go to when they’re not playing. So, it doesn’t bode well”.

Any outbreak during pre-season could also hamper players’ attempts to get match fit before the new campaign, according to Prof Robert Dingwall, one of the UK’s leading sociologists and a government adviser.

“Given the rise in cases, particularly among men under 30, which is a demographic strongly represented among Premier League footballers, clubs should expect to detect cases as their playing staff return from summer breaks,” he said.

“This is likely to have some impact on individual fitness and pre-season training that could only be partially mitigated by testing. Players are unlikely to have experienced a serious illness but training regimes may need to be adjusted to allow for recovery.

“If infection comes into a squad, it could have a significant impact on the induction of new members or the introduction of new methods of working, as in any other business.

“It would be surprising if some clubs did not find they were going into the new season less well-prepared than they would like.”