Ambitious mass testing plans that could keep hopes of Christmas parties alive would cost as much as £100 billion, according to reports.

Leaked documents seen by the BMJ suggest that the "Operation Moonshot" project - which would see millions of UK-wide tests carried out daily - could have a price tag close to that of the £114 billion budget given to NHS England in 2018/19.

Boris Johnson believes the scheme could help sport and entertainment venues reopen fully and allow people to socially mix in large groups again with on-the-day tests.

The Deputy chief medical officer, Dr Jenny Harries, said the success of Moonshot would depend on how it is handled.

She told ITV's Peston: "We do want to get back to as much normality as we can and any opportunity to do that through a new testing programme or using different testing technology is clearly a good thing to be following, but it's not quite as simple as just doing that."

Moonshot has received mixed reviews from the scientific community, including from Dr Chaand Nagpaul, council chairman of the British Medical Association (BMA), who said it was unclear how it would work given the "huge problems" currently seen with lab capacity.

Ministers have faced increasing pressure in recent days over availability of tests, with many people reporting being sent hundreds of miles from home to get checked for the illness.

The new testing plans come as the Prime Minister admitted it was "too early to say" whether big parties could be held over the festive season this year.

Speaking at a Downing Street briefing on Wednesday, the Prime Minister hoped that "we could be able to get some aspects of our lives back to normal by Christmas" and pointed to mass testing as something which could help society open back up further.

For now, however, Mr Johnson urged people to limit their social contact "as much as possible" as he confirmed that social gatherings of more than six people will be outlawed in England from Monday.

Lockdown rules could also be tightened in other parts of the UK, as rates of Covid-19 infection have risen across the country in recent days.

Meanwhile the latest figures in the Government's test and trace will be released on Thursday.

Last week, the number of close contacts of people who tested positive for Covid-19 being reached through Test and Trace was at its lowest since the system was launched.

On Wednesday, a further 2,659 positive cases were confirmed across the UK, taking the total up to 355,219.

Positive results have increased from from 12.5 per 100,000 people to 19.7 per 100,000 in the UK in the last week - with a particular rise in infections among young people.

Infections are most prevalent among the 19 to 21-year-old age group, with 54 cases per 100,000 people.

In Northern Ireland, it is expected that restrictions will be tightened on Thursday, while in Scotland, First Minister Nicola Sturgeon will announce the latest review of lockdown rules north of the border.

Ms Sturgeon had already warned her latest review was "likely to take a very cautious approach".