Sixteen hotel rooms were examined by people acting as guests
Mouldy mattresses, stained duvets and dirty toilets have been found at budget hotels by a consumer rights magazine.
Which? Holiday magazine said it found poor cleaning standards at some London and Manchester hotels run by budget chains Ibis and Travelodge.
The dirtiest toilet was at Ibis Euston in central London, it said.
Spokesmen for Ibis and Travelodge said the levels of uncleanness found did not present a health risk to customers, but they would act on the investigation.
Sixteen hotel rooms were examined in September by a researcher for the magazine and microbiological technical consultants, who were pretending to be ordinary guests.
The magazine rated rooms in hotels run by the Comfort Inn and Jurys Inn chains as clean.
It also rated three Premier Inn hotels in Manchester as clean but two in London had "room for improvement".
Mould was found growing on a mattress at an Ibis hotel on Manchester's Charles Street, while a duvet at the Portland Street Ibis, also in Manchester, had a stain suspected to be blood.
A toilet at Ibis Euston in London was found to have urine and faeces around its seat and urine down its pedestal, the report said.
There were also concerns about cleanliness, such as levels of bacteria and dust, at three Travelodge hotels in London and two in Manchester.
Which? Holiday editor Lorna Cowan said: "Although this investigation was just a snapshot, it does raise concerns about the cleanliness of some budget hotel chains.
"It's clear from our research that some of the hotels are getting it right when appropriate cleaning methods are being used.
"Paying guests should be guaranteed, at the very least, a clean room."
Ms Cowan called for "one single accepted standard" for hygiene in hotels across the UK.
An Ibis spokesman said the chain had taken "immediate remedial actions" to rectify the "totally unacceptable" lapses in housekeeping standards found by the magazine.
"Ibis treats matters of cleanliness and hygiene as critically important.
"We have submitted the results that Which? obtained to an established independent health and safety consultant, who has confirmed that nothing in those results constitutes a danger to public health."
A Travelodge spokesman said levels of bacteria found at hotels examined by the magazine would not have presented a health risk to customers.
He added: "We take room hygiene standards extremely seriously and have taken immediate action to remedy all of the issues raised by the Which? report."
Premier Inn chief executive Paul Flaum said: "We take cleanliness and hygiene very seriously.
"The spotless standards achieved in our three Manchester sites are what we expect across all of our 550 hotels."