Civil liberties campaigners are concerned that the public could be “coerced” into sharing data about their movements through a coronavirus contact-tracing app being developed by the UK government.

The health secretary, Matt Hancock, says the app, which will be available within weeks, is “crucial for holding down the rate and the level of transmission” of Covid-19.

But the campaign group Liberty is worried that signing up to the app could become a compulsory condition for returning to work and being allowed out of lockdown when conditions are eased. Such a condition would amount to coercion, Liberty claims.

The academic developing the app has said that tens of millions of people would need to sign up to the app to make it effective.

Clare Collier, the advocacy director at Liberty, said: “By presenting surveillance tools as a solution to lockdown, the government is drawing on the willingness we have all shown to make sacrifices in the face of this crisis while refusing to show it is taking seriously the enormous risks presented by invasive technology.”

She added: “Contact-tracing technology may require us to sacrifice deeply sensitive personal information and there can be no question of state bodies or private companies sharing our personal data other than what is absolutely necessary. It is vital no one is coerced into having the app installed, and using it as a condition for returning to work or everyday life will inevitably lead to discrimination.

“The opportunities presented by technology should be fully investigated, but seeking quick solutions in surveillance tools that rely on our personal data creates serious long-term threats to our rights and ways of life.

“Contact-tracing technology will only work if 60% of people use it. But people will only use it if they trust their rights are being protected. For this approach to have any hope of helping us beat the virus, privacy safeguards must be hard-wired into the design from the outset.”

Prof Christophe Fraser, of the University of Oxford, who is leading a team developing the app, said it would a require a very high take-up to be effective.

“For this intervention alone to stop a resurgence of the epidemic, about 60% of the population would have to use the app,” he told the BBC. “The app is going to be one of the building blocks of how to get out of the epidemic. For every one to two users who download the app and who adhere to the instructions, you will prevent one infection.”

He that if a person became infected, the app would send warning messages to anyone who had come within Bluetooth range of their phone. He said: “You’ll either get a message which says you’ve been in contact with a suspected case of Covid-19. Or you may later get a message and you’ve been a contact of a confirmed case. At that point, we would recommend quarantine of 14 days after the contact has taken place.”

He added: “When you install the app, this is an opt-in system so it will work in the background. Your phone builds up a memory of the sort of anonymous IDs of the other phones you come into contact with. It needs to be a contact long enough that there’s a risk of transmission.”

Asked about privacy concerns he said: “The NHS records patient data already and we trust the NHS to look after private data. This is being developed with a minimal amount of data – enough to make it work. It’s a public health intervention with data collected within a health system.”