Britannia plumbed new depths with a one-star rating in most categories, including cleanliness, in this year's survey of large UK hotel chains.
The budget brand was described as a 'dirty hovel' by one guest, landing the bottom spot for the eighth consecutive year.
Sofitel, part of Accor, came top with an impressive customer score of 86%. The luxury hotel group - which has properties at Heathrow, Gatwick and London St James - knocked Premier Inn off the top spot for the first time in five years.
It suggests that many guests are willing to spend more on hotel stays this year amid the global pandemic.
This year saw Britannia muster an overall score of just 37%; with one-star ratings for the quality of its bathrooms, bedrooms and communal areas. It was also rated one star for value for money despite being one of the cheapest brands in our survey, at £58 per night on average.
Our undercover inspectors found stained towels, stray hairs and pillows 'flimsier than bookmarks' when visiting its Folkestone chain, the Grand Burstin, this summer.
A demonstrated worrying lapses in cleaning between guests. The situation at Britannia's Brighton seafront hotel, the Royal Albion, wasn't much better. Swab tests revealed traces of enterococci, bacteria found in faeces, on the toilet seat and bathroom door handle.
This was despite the chain making promises of enhanced cleaning to combat the spread of coronavirus.
A spokesperson for Britannia told us: 'We are totally committed to providing a safe environment for visitors. We have so far spent around £2 million on COVID-19 precautions, but we accept there is more to do.'
Mercure and Days Inn hotels were also left languishing near the bottom of the table, with scores of 60% and 63% respectively. But when it came to cleanliness, our swab tests came back without any cause for concern.
Watch the grim results of our germ simulation powder tests at a Britannia Hotel.
Sofitel was this year's success story, with five stars in most categories - including the quality of its bedrooms and bathrooms.
The majority of guests said the premium price tag, £148 a night on average, was worth it for its 'outstanding service' and 'impeccable cleanliness'. One guest described it as 'probably the best hotel room I have ever occupied'.
However, our undercover inspector found low levels of faecal enterococci on the toilet seat when swabbing our Superior room at the Sofitel St James.
A spokesperson for the chain said it believed this to be an isolated issue, but had conducted a 'thorough review of its cleaning protocols' in response to the findings.
Second place Premier Inn scored a solid 82% this year and was praised for its reliable quality at a reasonable price.
Guests described the rooms as clean and comfortable, with countless respondents telling us 'you know what you're getting'.
Both Sofitel and Premier Inn were named Which? Recommended Providers, alongside Holiday Inn Express and Hilton Hampton. All four brands have been treating customers well during the pandemic; refunding bookings that were unable to go ahead as planned.
Warner Leisure and Hotel Du Vin came out on top for small/medium chains, with both being named Which? Recommended Providers.
We asked more than 4,000 Which? members and members of the public to rate their experiences at large and small/medium UK hotel chains.
Guests scored them on a range of criteria; including the quality of bedrooms and bathrooms, cleanliness, customer service and value for money.
Our undercover inspectors also visited six hotels during August, from the chains that performed the best and worst in our survey. We tested the hygiene by swabbing eight high-contact areas in the room, including door handles and light switches.