Scores of holidaymakers, including the author Michael Rosen, have had their trips cancelled at the last minute after Europe’s leading holiday rentals group re-let the accommodation they had booked to new customers.

Customers of Cottages.com and Hoseasons, both part of the Awaze Group, have discovered that their bookings have been cancelled, often at the last minute and sometimes after arriving at their accommodation.

The writer Michael Rosen, who spent three months in hospital with coronavirus, had booked a newly listed property in Somerset for a week with his family.

Four days before the holiday, an email arrived saying the reservation had been cancelled.

“They gave no reason, did not answer their phone or respond to emails,” said his wife, Emma-Louise Williams. She was eventually told, via Twitter, that the owner had withdrawn the property. However, the owner claimed Cottages.com had listed it on the website when it had already been booked.

“It is very disappointing,” Williams said. “Michael had been really looking forward to a week away because he has been so seriously ill and just needed a moment to breathe.”

Cottages.com reported a 455% increase in bookings when lockdown was eased in June, and the company told the Guardian that the huge increase in demand had caused technical problems.

Some of those affected lost out after being told that their balance payments would be delayed until mid-July and debited automatically from their credit card.

They were not informed when Cottages.com failed to collect the money and their bookings were resold.

Clare Moreton discovered that she would not be holidaying in the Scottish Highlands when she contacted the owner of her cottage and was told her booking had been cancelled.

Cottages.com had failed to debit her balance payment as agreed and she had been unable to get through to pay it manually. “My holiday is due to start in five days and I’ve still not been informed by Cottages.com that it’s been cancelled,” she said.

One customer had booked a villa in France last December and spent three days trying to contact the company after noticing that the balance payment had not been taken last month.

He was eventually informed that a system error was preventing payments but that the booking was safe and he would be called within 24 hours to take the balance. The call never came.

He discovered from his contract that the reservation had been made through a sister company, Novasol, which told him it had been cancelled by Cottages.com the day before he had been assured it was unaffected.

“My wife is a key worker who really needed the break and we had already paid for other parts of the holiday so I had no choice but to book a property through another company,” he said. “It cost double the original because prices have soared since the pandemic so I’m out of pocket by £6,000.”

Cottages.com refunded his cancelled booking and contributed £5,000 to the replacement after the Guardian intervened. Rosen’s family was offered £250 compensation on top of a refund after the Guardian contacted the company.

Awaze UK, which owns more than 20 holiday rental brands, blamed unprecedented demand for overwhelming its systems and a technical error that prevented balance payments being collected last month.

It denied it had cancelled longstanding bookings to sell on at a higher price.

“Though we expected demand to resurface as UK travel restrictions were lifted, we didn’t plan for a tenfold increase, which is what we’ve experienced on some of our platforms in recent weeks,” said the chief executive, Henrik Kjellberg.

“Our systems didn’t scale to the level we needed them to and this regrettably caused some duplicate bookings to occur.”

The company said all affected customers would be offered alternative accommodation at the same price or a refund if there was no availability, but customers who have contacted the Guardian say that they were not helped with rebooking and are awaiting a refund.

Awaze was bought by a private equity firm for $1.3bn (£1bn) in 2018 and manages more than 110,000 rental properties across Europe.

It was investigated by the Competitions and Markets Authority (CMA) earlier this year after refusing to refund customers whose bookings had been cancelled due to lockdown. In June it announced that it would reimburse affected customers after the CMA ruled that it may have breached consumer law.

The company has also received a warning from the reviews website Trustpilot after the Guardian reported a spate of five-star ratings by “invited” customers. Firms that subscribe to Trustpilot’s Enterprise Account can provide the platform with the contact details of customers they want to be approached for a review.

Cottages.com and Hoseasons began inviting reviews in March when negative headlines over their refunds policy began. Nearly 80% of the invited reviews rate the companies as excellent or good compared with 8% of spontaneous reviews whereas the figures are reversed for uninvited reviews.

Trustpilot, which admits that invited reviews are the biggest driver of a high score, said both companies had breached its guidelines.

“Unfortunately, it has turned out that the companies have only invited those who had successfully rebooked,” said a spokesperson.

“To be truly fair and unbiased in their approach they should also have invited those who may not have successfully rebooked. We are working with them at the moment to ensure that our guidelines are being met.”

Awaze said it invited reviews in good faith from customers whose holidays had been cancelled to monitor the effectiveness of its online transfer option.