Airlines have been accused of “harmful practices” in their treatment of passengers affected by disruption.

The Competition and Markets Authority (CMA) and the Civil Aviation Authority (CAA) have issued a joint letter to carriers, expressing concern that “consumers could experience significant harm unless airlines meet their obligations”.

The letter stated: “We are concerned that some airlines may not be doing everything they could to avoid engaging in one or more harmful practices.”

These include selling more tickets for flights “than they can reasonably expect to supply”, not always “fully satisfying obligations” to offer flights on alternative airlines to passengers affected by cancellations, and failing to give consumers “sufficiently clear and upfront information about their rights”.

The letter said the CMA and CAA are “considering evidence about these issues” and “share consumer protection law enforcement powers” in the aviation sector.

Tens of thousands of flights have been cancelled this summer as the industry struggles to cope with the demand for air travel amid staffing shortages.

Many passengers have been left frustrated by how airlines have acted after axing flights.

The CMA and CAA said they expect airlines to “not continue marketing tickets for flights if they cannot be reasonably confident they will go ahead”.

After a flight is cancelled, airlines unable to offer a “timely replacement” flight must give passengers the option of flying with another carrier, according to the letter.

But some companies ask passengers to make their own arrangements in these circumstances.

The letter stated: “We have concerns that in some cases, this is likely to breach professional diligence standards for those consumers who are not in a position to do so.

“For example, those who may be unable to: investigate or book alternative routes; self-fund the purchase of flight tickets and accommodation; or to afford to wait for reimbursement, would not be able to benefit from their statutory rights in the event of flight cancellation.

“We urge airlines operating this practice to quickly put in place mechanisms for these consumers to ensure re-routing is a viable option for them.”

The CMA and CAA also said passengers’ rights must be “presented clearly”, and consumers “should not be required to hunt for such information”.