Experts have stressed the importance of acting fast to tackle the spread of Omicron as an NHS boss warned that pressure on trusts in London is “mounting rapidly”.

It comes as 1,400 new Covid cases were reported in Berkshire on Saturday.

A number of scientific advisers to the Government spoke on Sunday, a day after minutes were published from a meeting warning it was “almost certain” there were hundreds of thousands of new Omicron infections each day in England.

The update from the Scientific Advisory Group for Emergencies (Sage) also stated that UK hospital admissions with the variant were probably around one-tenth of the true number because of a lag in hospital reporting.

Sage member Professor Sir David Spiegelhalter said the timing was crucial.

He told Trevor Phillips On Sunday on Sky News: “We can’t wait for hospitalisations to go through the roof before we do something about it, because by then it’s too late.

“And in fact what happens by the end of this year is pretty well built-in now. There’s almost pretty well nothing we can do about it.”

He said boosters would not have a “big effect” on the immediate future because of the time it will take to build immunity.

On measures, he added: “Every day we delay will have a bigger impact further on.”

He said he was glad not to be the one making decisions “because they’re extremely difficult ones”.

Professor Mark Woolhouse, a member of the Scientific Pandemic Influenza Group on Modelling (SPI-M), said while there remained uncertainty around the severity of the Omicron variant, it was right that plans were being made in the event of a “tsunami” of cases as described by Scotland’s First Minister Nicola Sturgeon.

He told Times Radio: “I think it’s absolutely right that governments and administrations around the UK are planning for that possibility as they are doing.”

He added: “We can’t afford to wait, but there is still the decision to make about what level of interventions.”

He said there was a “difficult balancing act”, but added: “I’m personally in favour of doing everything you can to make contacts as safe as possible, rather than banning them altogether.”

However, he said the scientific advice at this stage suggested that the Plan B measures and changes in the behaviour of the public probably would not be enough “to bring this wave completely under control”.

Chris Hopson, chief executive of NHS Providers, said there was still uncertainty, but information to inform whether Omicron will result in significant hospital admissions should be available soon.

He told Times Radio: “We need to be ready to very, very quickly implement tougher restrictions should there be very clear evidence that Omicron is going to lead to significant levels of hospitalisations and as I said, we should know that information in the not-too-distant future.”

He said London trusts in particular were facing pressure from rising Covid hospital admissions.

He said: “Over the last week, the pressure on London trusts has been mounting rapidly. It’s not just hospitals, it’s community mental health and ambulance trusts, too.

“So, for example, the number of hospitalised Covid patients or patients who tested positive for Covid has gone up by 30% in a week, at a time when nationally it’s only gone up by 4%.”

Professor Stephen Reicher, a member of Scientific Pandemic Insights Group on Behaviours (SPI-B), said Omicron’s faster transmissibility means it is “coming at us like an express train”, as he called for clear messaging to the public.

He told BBC News: “A good clear message is more important now than ever before, of how serious the crisis is.

“We’re in a position now, where the new variant Omicron is doubling, possibly in less than two days. That means that if you wait two days, you have twice as big a problem and you have to do twice as much to bring things under control. So the urgency of the situation needs to be communicated, it seems to me, with great, great clarity.”

He added that “good information from the Government, combined with good support from the Government” would likely lead to people accepting “the measures that are necessary to bring this thing under control”.