Norwich are waiting anxiously for Covid-19 results ahead of their clash with Manchester United after Premier League sides were told to return to emergency protocols to limit the spread of the virus.

Tottenham's Europa League Conference game with Rennes on Thursday was postponed after Spurs reported that eight players and five members of staff had tested positive.

Sunday's match between Brighton and Tottenham is also off as fears rise over the potential impact of the surging Omicron variant on the English top-flight. Spurs' trip to Leicester on Thursday is also in doubt.

Leicester were without seven players due to positive cases or other illnesses for Thursday's 3-2 defeat at Napoli, which knocked the Foxes out of the Europa League.

Norwich manager Dean Smith, whose side lost 3-0 to Tottenham last week, faces a nervous wait to find out if any of his players will be ruled out of the United match because of a positive test result.

"We have got a couple of knocks and have got a couple of Covid-related issues that we are waiting on results for this morning," he told his pre-match press conference on Friday.

"I can't really let you in on any names, but there's a couple of players who we are worried about, so we have tested and we will wait for the PCR results back on them.

"(They are) just feeling a little bit unwell, not likely to be linked to the game last week with Tottenham. It is just in general, I think (like) with the whole nation, (we are) holding our breath at the moment."

The Premier League has written to its 20 clubs returning training ground protocols to "emergency measures".

It followed an announcement this week by British Prime Minister Boris Johnson that the government was tightening virus restrictions in England.

The UK as a whole has had more than 10 million confirmed cases of coronavirus and more than 146,000 people have died from the virus, one of the highest tolls in Europe.

Rules had been relaxed at Premier League clubs with high vaccination rates, allowing players to remove masks indoors and use their original dressing room.

"We will try to be as cautious and careful as possible and do all the things," said Liverpool manager Jurgen Klopp.

"It might be an extra test per week. With Champions League games we get tests twice a week anyway -- now we'll probably get tested three times a week which I'm completely fine with."

Clubs have also been advised to limit social interactions, meaning Christmas parties are likely to be off the table.

Manchester City's training ground was shut down by an outbreak of cases just under a year ago.

Manager Pep Guardiola said his players were still on "alert" to prevent a similar outbreak at the busiest time of the year for Premier League clubs, with six rounds of fixtures scheduled to take place between this weekend and January 3.

"The concern is always here -- the variant or the pandemic is still in society," he said.

"We follow the protocols. Everyone is alert. What happened with other clubs can happen here, absolutely. Wear masks, social distance, be careful."

As part of the new measures announced this week, fans will have to show proof of full vaccination or a negative test to attend sporting events with crowds of more than 10,000 from next Wednesday.

The coronavirus pandemic largely forced football behind closed doors last season but full crowds have returned to English football during the current campaign.

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MANCHESTER UNITED