Any relaxation of restrictions over the festive period will result in a “large” rise in coronavirus transmission rates and prevalence “could easily double”, scientists advising the Government have said.

In a set of documents released on Friday, the Scientific Advisory Group for Emergencies (Sage), which advises the Government, said “substantial mixing” of people over a short period of time represents a “significant risk for wide-spread transmission”.

In a meeting on November 18 – a few days before the Government announced that there would be a limited relaxation of restrictions at Christmas – the scientists warned: “The prevalence could easily double during a few days of festive season, with further multiplicative increases as new infections go back to their ‘routine’ networks.”

This week Boris Johnson told families they must make a “personal judgment” about the risks of coronavirus to vulnerable loved ones when forming a Christmas bubble.

The Prime Minister urged the public to “think carefully” over the festive period after it was confirmed that three households will be able to mix from December 23 to 27.

In a meeting on October 29 – a week before England went into a second national lockdown – Sage warned that to reduce the “inevitable risk from social mixing during the festive period”, a substantial reduction in prevalence is required ahead of any changes to behaviours or interventions.

In a summary of the meeting, it added that measures to achieve this would need to be put in place “as soon as possible”.

Since then, the Government has announced the introduction of a tougher tier system in England at the end of the second national lockdown on December 2 which will see more than 55 million people placed into Tier 2 and Tier 3 measures.

Meanwhile, experts advising the Government have said that, based on evidence from other respiratory infections, elder family members are generally at increased risk of respiratory diseases during the festive period with younger people contributing to this risk.

They added that Sars-Cov-2, the virus that causes Covid-19, has shown “high secondary attack rates in households – with estimates of up to 50% in one household becoming infected from one infected member”.

In a document dated November 26, the Independent Scientific Pandemic Insights Group on Behaviours (SPI-B), which advises Sage, said interactions in a home environment are likely to pose greater risks than public and workplace settings.

This is due to familiarity with the location and people, and a lack of clear responsibility to ensure the environment is safe.

The experts said: “People are more likely to relax and let their guard down with those they are closest to.”

They also said allowing households to “bubble” during such the festive period – where a group of people effectively form a single, larger household which does not mix with others – may be better at keeping infections under control than limiting contacts to a specific number.

But the experts said for that proposal to work, “faithfulness to bubbles is paramount”.

According to the Government guidelines, each Christmas bubble can meet at home, at a place of worship or an outdoor public location, but existing, more restrictive rules on hospitality and other venues will be maintained throughout the period.

The experts said: “Bubbling would be most effective if time-limited to less than one generation time of infection (about one week).”

They added: “The larger the number of households in the bubbles, the greater the risk of ‘extra-bubble’ contacts and transmission risk.

“Whatever system is chosen, sequential or multiple bubbling should be avoided.”

Meanwhile, in another undated document, the SPI-B warned that one of the primary challenges of Christmas celebrations will be enforcement.

The document said: “Enforcement of celebration behaviours in private homes is almost impossible.”