The Competition and Markets Authority has written an open letter to test firms warning of its concerns. They include:

* Advertising upfront prices for PCR tests which do not include additional charges everyone must pay.

* Advertising cheap PCR tests which are only actually available in very small quantities or are not available at all.

* Failing to deliver PCR tests or provide results within stated timescales, or at all.

* Refusing to provide refunds where tests are not provided within advertised and/or agreed timescales, or at all.

The government has separately this week warned 82 companies they face being removed from the gov.uk list if they advertise misleading prices.

The CMA instructs PCR test providers to “immediately review their practices and policies to make sure they are in line with the requirements of consumer law and to make any changes where necessary” and warns of legal action if they do not.

CMA general counsel Sarah Cardell said: “PCR test providers should be in no doubt that they need to get on the right side of the law. If they don’t, they risk enforcement action.

“Our advice today will also help people by setting out exactly what they should expect for their money.”

The letter lists 11 steps providers should take, including not focusing their advertising on cheap tests which are only available in small numbers, showing the full cost of tests including all compulsory charges and providing “honest, accurate and clear” timescales on when tests will be received.

PCR test providers must also ensure PCR tests and results are provided within advertised timescales.

The CMA added: “We continue to work closely with the Department of Health and Social Care in reviewing this market and will be providing further advice to DHSC on action that can be taken.”