Some hotels failed to adequately sanitise every room despite claims of enhanced cleaning during the pandemic, a Which? Travel investigation found.
Using germ simulation powder and swab tests, our secret inspectors uncovered poor hygiene when we visited two Britannia properties, the Grand Burstin Hotel in Folkestone and Brighton's Royal Albion.
Britannia - which claims rooms are 'freshly sanitised' to limit the spread of COVID-19 - has been rated the UK's worst hotel chain for the eighth year running in our latest survey.
After lockdown this summer, we booked the same roomunder different names for two consecutive nights atthe Folkestone Britannia.
Before checkout, we sprayed towels and bedsheets with Hazmat UV fluid (used by the Met Police for crime scene training) to check how thoroughly rooms were cleaned between guests.
We also sprinkled germ simulation powder (used for NHS demos) on common touch areas, including the light switches and door handles.
Both washable substances are invisible to the naked eye, but glow red under an ultraviolet light.
When we checked back in, many items lit up under our UV black light - including the TV remote control, door handle, plug socket, kettle and coffee mugs. In the bathroom, the bathroom taps, soap dispenser, toilet brush and flush also turned red. Fortunately, the towels and bedsheets had been changed.
A decent cleaning fluid should have removed the powder from all surfaces. It's especially worrying when COVID-19 can live on surfaces for up to 72 hours, according to The New England Journal of Medicine.
A spokesperson for Britannia told us at the time: 'We are totally committed to providing a safe environment for visitors. We have so far spent around £2m on COVID-19 precautions, but we accept there is more to do.'
Britannia is signed up to VisitEngland's 'We're Good to Go' industry standard. The logo is displayed on its website, confirming its hotels follow government and industry COVID-19 guidelines on cleanliness and social distancing.
Britannia has made changes - with hand sanitiser stations at the entrance and plexiglass in the lobby. A card inside our room also promised: You can be confident that your bedroom has been thoroughly sanitised ready for your stay.
But even before our experiment, we found stray hairs and a towel covered with brown stains.
So it's no surprise that the chain mustered a one-star rating out of five for cleanliness in this year's Which? Travel survey. Britannia once again ranked bottom with a dismal customer score of 37%.
Pre-pandemic, one member reported finding a blood-stained tissue in the holder on check-in, while another branded it a 'filthy hovel'.
Our undercover inspectors visited a further five hotels during August, for chains which performed the best and worst in our survey.
We found more signs of sloppy cleaning at Britannia's Brighton seafront property, the Royal Albion. This time we used a swabbing kit to collect samples from eight high-contact areas in the room.
Lab tests revealed traces of enterococci, an indication of faecal contamination, on the toilet seat and bathroom door handle.
Low levels of staphylococcus were also found on the TV remote, bathroom door handle and toilet seat.
Sofitel was the only brand in our survey to receive full marks for cleanliness. Yet our undercover inspectors still found traces of faecal enterococci on the toilet seat at its £300-a-night London St James' property.
Levels were low, but it still points to inadequate cleaning. A spokesperson for the hotel said they were 'surprised and disappointed' by the results.
They added: 'While we believe this to be an isolated issue, we have undertaken a thorough review of the hotel's cleaning protocols.'
Others, such as the White Horse in East Sussex - owned by the Greene King Old English Inn chain - performed better. Swab results were spotless, despite its cheaper room rate of £92 (including breakfast).
A spokesperson for the chain confirmed that it was disinfecting all contact surfaces and washing towels and linen with antiviral detergent.
It also had strict protocols around breakfast, with pre-booked slots and a one-way system to prevent overcrowding. Signs also indicated that tables had been 'freshly sanitised'.
It was a stark contrast at Britannia's Grand Burstin Hotel. The push-button hot drinks machine was still self-service and covered in puddles of tea, spilt sugar granules and empty sachets. Plates of leftover food were also piling up on tables.
This is despite government COVID-19 guidelines insisting upon 'frequent cleaning of objects and surfaces that are touched regularly' - including disinfecting tables between each and every guest.
Both the Grand Burstin Hotel and the White Horse are verified by VisitEngland's We're Good to Go scheme: an industry standard to reassure guests that government and Public Health guidance is being followed.
Hotel managers need only complete a self-assessment form to be accepted. Tick every box and you can display the endorsement on your website.
Andrew Stokes, VisitEngland's director, told us that all verified businesses must have a COVID-19 risk assessment in place. The tourist organisation also carries out random spot checks.
But when we visited in late summer, only 151 of the 7,000 listed hotels and B&Bs had been inspected.
As a result of our complaint, VisitEngland told us it sent an assessor to the Britannia Grand Burstin Hotel, but found no reason to remove it from the We're Good to Go scheme.
More robust is Quality in Tourism's (QT) Safe, Clean & Legal - a scheme that launched in 2018, but has been updated to include COVID-19 guidance. The application form requires detailed answers and all properties are inspected by an independent third party.