Whirlpool, the firm behind the faulty tumble dryers involved in a series of fires, has been accused of "multiple failings" by the consumer group Which?
It said the US company is withholding information from customers, and making them wait too long for repairs.
Which? said some owners were having to wait up to 12 weeks, just to obtain a customer ID.
The company said there were inconsistencies in the report, but it would investigate the issues raised.
The BBC has previously reported that customers are having to wait up to 11 months in total for repairs to be carried out.
The fires have been caused by excess fluff, which can catch on the heating element. Some five million affected machines have been sold in the UK, under the Creda, Indesit and Hotpoint brands.
Which? used a series of mystery shopper calls to test responses from Whirlpool.
It found that customers were repeatedly told to take up the problem with the retailer who had sold them the machine.
When they did so, consumers were re-directed back to Whirlpool.
Many owners were faced with the choice of lengthy delays for a repair, or else paying for a replacement at a reduced price.
Some were told by Whirlpool staff that it would take eight to twelve weeks just to be given a customer ID - something Which? said could have been done at the touch of a button.
"Whirlpool customers rightly feel dissatisfied with how they've been treated, being faced with delays, confusion and a lack of information," said Alex Neill, director of policy and campaigns at Which?
"Whirlpool must clean up their act and sort this mess out," she said.
Which? has also called on Whirlpool to list all 127 affected model numbers on its website, rather than the model checker which is currently available.
In response, the company said there were "a number of inconsistencies" in the Which? report.
However it promised to investigate any instances which were inconsistent with its "high standards."
In a statement it said, "Whirlpool's response to the tumble dryer issue is at an unprecedented level, and our staff are working round-the-clock to ensure we're able to resolve the matter as quickly as possible for our customers."
It was continually looking at ways of speeding up the repair process, it said.
However Which? is calling on the government to intervene, to close the "loopholes that allow companies to leave consumers without the basic information and advice they need."