Whirlpool has admitted that there could be as many as 800,000 faulty tumble-dryers in homes around the UK.
In June, the government said it would issue a recall notice of up to 500,000 dryers which pose a fire safety risk.
But when pressed by MPs on the Business Committee, company executives admitted the number of unmodified machines could be higher.
A fault in Whirlpool machines was blamed for at least 750 fires over an 11-year period, the government said.
Whirlpool said it had logged 54 fires caused by a build-up of fluff in its tumble dryers in recent years, three of which were in machines that had been modified.
Charlie Pugsley, deputy assistant commissioner at the London Fire Brigade, said his service had seen a wide range of faults causing fires in machines that had already been modified.
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Jemma Spurr was one customer whose modified dryer caught fire.
She told the Business Energy and Industrial Strategy Committee that despite repeated attempts to get in touch with Whirlpool directly, she had never received the report on the cause of her fire, or an apology from the company.
Whirlpool executive Jeff Noel apologised to Ms Spurr during the hearing and said the company had modified every machine bought to its attention.
Ms Spurr also claimed that Whirlpool asked her to sign a non-disclosure agreement (NDA) about the incident. She did sign it but has spoken out anyway.
Whirlpool said non-disclosure agreements were standard industry practice during insurance settlements.
By Colletta Smith, BBC consumer affairs correspondent
Faulty dryers, months to wait for a fix, continued problems even after modifications, and a full product recall. It’s a string of bad publicity for a company trying to brand itself as a provider of “quality home appliances”.
It isn’t the first time that the company’s corporate vice-president has had to defend his products to a parliamentary committee. Jeff Noel previously responded to safety concerns about the company’s fridge-freezers which were blamed as the cause of the Grenfell fire.
Customers are understandably frustrated, and the white goods market is particularly dependent on trust.
Research by Deloitte suggests that people are more likely to read online reviews, and ask family and friends for recommendations of household appliances than any other purchase.
The string of damaging news has put Whirlpool in a real spin, as it comes at the same time as increased competition from the likes of Samsung, Bosch, Siemens and Zanussi.
The company also confirmed that, during the recall period, customers can either have their current dryers modified or get a brand new machine free of charge, including installation.
In a statement, the Whirlpool Corporation told the BBC: “The Office for Product Safety and Standards (OPSS) recently concluded a comprehensive year-long review of the dryer programme which confirmed that the modification is effective in resolving the issue.
“Safety is always our number one priority and we remain committed to resolving all unmodified dryers affected by this issue.
“As we updated the committee, we are expanding our recall campaign to include further options to encourage remaining consumers to come forward and remedy their unmodified appliances.
“The crucial message to anyone who still owns an affected dryer and has not already had it modified by Whirlpool is to contact us immediately on 0800 151 0905, or visit https://safety.hotpoint.eu/, https://safety.indesit.eu/ or https://safety-swan.eu.
“As advised by OPSS, consumers whose tumble dryers have been modified can continue to use them safely and there is no need to contact Whirlpool at this time.”
Sue Davies, strategic policy adviser at consumer group Which?, said: “With Whirlpool admitting it has only managed to provide a modification or replacement for a tiny proportion of affected machines in the last two years, it’s clear that the company is failing to do enough to keep customers safe.
“Now it has acknowledged that modified machines are still catching fire.
“If the safety of Whirlpool’s fire-risk tumble dryers cannot be assured, secretary of state Greg Clark must step in and ensure that all potentially dangerous machines are immediately removed from people’s homes.”