The sharp rise in the number of people being told to self-isolate after receiving notifications from the NHS Covid app threatens to put millions of holidays in jeopardy.

Consumer group Which? warned it can be “incredibly difficult” for those affected to get their money back.

The UK’s two largest tour operators, Tui and Jet2holidays, allow customers pinged by the app to make fee-free changes.

That means the normal admin fee is waived, but holidaymakers are still being charged for any increase in the price of their new booking.

Which? said just a handful of firms such as Kuoni, Exodus and Explore have pledged to issue refunds for Test and Trace disruption.

Airlines generally have policies which are more restrictive.

Ryanair only allows fee-free changes up to seven days before a flight.

That means passengers who cannot fly due to being told to self-isolate within a week of departure are charged rebooking fees ranging from £35 to £95 per flight, depending on the route and travel dates.

This is on top of any increase in air fares for the new itinerary.

EasyJet customers can make fee-free changes up until two hours before a flight’s departure.

British Airways has a similar policy, offering passengers the option of rebooking without a fee or requesting a voucher up to the time check-in closes for a flight.

Meanwhile analysis of 263 travel insurance policies by Which? found only around one in six include cancellation cover in the event of self-isolation being required.

The NHS Covid-19 app covers England and Wales only, with similar contact tracing apps in place in Scotland and Northern Ireland.

The app instructs people to self-isolate for up to 10 days if they have been in close contact with someone who has tested positive for coronavirus.

More than 530,000 of these alerts were sent in England and Wales during the first week of July.

But the request to self-isolate is not legally enforceable, meaning someone who has been pinged could go on holiday without breaking any laws.

Which? Travel editor Rory Boland said: “With restrictions easing and cases rising, it’s been suggested that several million people could be required to self-isolate this summer, either through being contacted by Test and Trace or notified by the NHS Covid-19 app.

“It can be incredibly difficult to secure a refund if you have only been told to isolate by the app, as there are no airlines and very few holiday operators that will allow customers to cancel for their money back in this scenario.

“Some travel insurance policies do offer cover for being told to self-isolate, but it’s important to check the small print to understand the extent to which you are covered.”