It is the ninth consecutive year that Britannia has come bottom of the pile in the annual survey by consumer group Which?.
The chain has 61 hotels across Britain, including Blackpool’s Metropole and Norbreck Castle and Scarborough’s Grand Hotel.
It received an average customer score of only 49 per cent.
More than half (51 per cent) of Britannia guests surveyed said they ran into a problem during their stay, with cleanliness being the most common issue.
The chain was rated one star out of five for bathrooms, and two stars for seven other categories such as cleanliness, customer service and value for money.
One customer said: “It was terrible. The room was dirty. The bathroom was dirty. The carpet was terrible, stains everywhere.”
Others complained about hotels being “run into the ground” and “in need of a drastic makeover”.
The average price paid of those surveyed for a one-night stay was £99.
Mercure’s average customer score was only slightly better at 52 per cent, with many guests observing that its standards have fallen.
One consumer said the chain’s hotels are “not as smart as they used to be” and it has “some poor quality properties”.
It was rated just two stars for cleanliness, rooms, bathrooms, communal areas and value for money.
Mercure guests surveyed paid an average of £114 for one night.
Premier Inn, with an average price per night of £66, was the best performing large hotel chain with a customer score of 79%.
It was topped by the best small chain, Hotel du Vin, which scored 80%.
Its guests paid an average of £150 for one night.
The research features 24 large hotel chains and six small ones.
A total of 2,371 members of the public and Which? members who stayed in a UK hotel in the 15 months to October 2021 were surveyed.
This included 57 who had visited a Britannia hotel and 73 who had spent the night at a Mercure property.
Which? Travel editor Rory Boland said: “Year after year, guests are let down by Britannia’s run-down hotels and often dirty rooms.
“This year saw some slight improvements to the chain’s score – but not enough to drag it off the bottom of our rankings.
“Until the company ups its game further we would urge guests to look elsewhere.
“The impressive, budget-friendly Premier Inn is our pick of the large chains, and Hotel du Vin offers high quality stays in interesting locations.”
A spokesman for Mercure said: “We are surprised to see our position in this survey.
“It does not reflect the high standard of guest experience which we strive for, and we will take action to address these comments.
“The experience and wellbeing of our guests at each and every property is our highest priority.”
It added that it was ranked “amongst the top mid-scale brands” in the latest annual hotel guest survey by research consultancy BDRC.