Millions of Britons had their movements "unwittingly tracked" using their mobile phones to see if vaccinated people moved about more after their jabs, the Telegraph has learnt.
A report from the SPI-B committee of Government scientists admitted that data from one in ten peoples' phones were tracked in February, without their owners' express knowledge.
The figures were used by researchers at Oxford University, who carried out studies for the Scientific Pandemic Influenza Group on Behaviours (SPI-B), which advises the Sage group of Government scientific advisers.
Using “cell phone mobility data for 10 per cent of the British population”, the Oxford University experts found 4,254 individuals were vaccinated.
This group was then tracked through 40 “CDR [call data records] with corresponding location observations” every day, and monitored for behavioural change by analysing their data for the week prior and week after the vaccination event.
The report said that “various robustness checks are undertaken by age, distance from home to vaccination point” were checked.
The experts also looked at “gyration (radius of gyration on vaccination day), time (opening hours) and home (do they go home directly after vaccination)”.
By comparing the movements of the vaccinated people against a different control group the scientists found that their “average pre-vaccination mobility increased by 218 meters [sic]”.
Campaigners against greater state surveillance in the UK said Britons would be “disturbed to discover they were unwittingly tracked and subjected to behavioural analysis via their phones”.
Silkie Carlo, a spokesman for Big Brother Watch, said: "No one expects that by going to get a vaccine they will be tracked and monitored by their own Government.
“This is deeply chilling and could be extremely damaging to public trust in medical confidentiality.
"Between looming Covid passports and vaccine phone surveillance, this Government is turning Britain into a Big Brother state under the cover of Covid. This should be a wake up call to us all."
A Government spokesman said: "All the data sets used in this research are set out in the paper which makes clear that the mobile phone location data used is GDPR-compliant and has been provided from a company that collected, cleaned, and anonymized the data.
"The data is at cell tower rather than individual level and the researchers were granted access to the dataset under a research contract with ethical approval provided to the researchers from the University of Oxford, working on behalf of SPI-B".
A Government source said: "This analysis is at the cell tower level of anonymised data and is therefore not individual surveillance.
"This coarse resolution of data at the cell phone tower level can range from around 300 meters to 1-2 kilometres and even up to 12 kilometres in rural areas.
"Although the location is smaller in urban areas, the population density is also higher, meaning it is not possible to identify individuals.
"It is not GPS tracing data which is commonly used by some large commercial companies for targeted advertising."
The source added the project was approved by an Ethics committee at Oxford University, with the data "provided by the company as part of the Covid-19 response of sharing data for the public good.
"This data is incidental and automatically generated when people use their mobile phones and is part of the general terms and conditions.
"Data was extensively anonymised by the company before it is used for research.
"Users are given a new token of identification each month to preserve anonymity and the only basic demographic data that is shared is age within a two-year bracket and self-reported sex.
"Only a small group of pre-approved researchers had access to the data."