London Trading Standards said the majority of masks seized had been labelled with false claims or fake safety certificates and around 4.25 million had to undergo label amendments before they were released.
A total of 2.25 million masks were found not to comply with legal safety standards.
Trading standards teams examined a further 1.5 million face masks and no issues were found.
Many of the consignments were referred to trading standards by Border Force teams at the airport.
The teams also seized 8,000 fake hand sanitisers, branded Andrex and Comfort, at the airport.
Suspicions were raised as they had identical packaging and labelling, except for the brand name, and the same batch code on the entire consignment.
Identical fake sanitiser products have been found on sale in Bexley by trading standards officers, officials said.
A further 4,500 hand sanitisers with false labelling were seized at the airport, according to LTS.
As well as stopping dodgy imports, trading standards are increasingly concerned about the sale of unsafe UK-made hand sanitisers that fail to meet safety standards.
Tower Hamlets trading standards have recently found UK-made hand sanitiser on sale online from a local shop that contains the banned substance Triclosan and Ealing Council’s trading standards team recently detained 454,500 face masks described as “anti-virus” where fake safety certification was supplied.
Ealing also seized 60,000 face masks that failed to have the necessary importers’ details, batch, declaration of conformity or test certificates available to demonstrate compliance.
The borough also seized 3,390 hand sanitisers, which lacked any legally required information regarding ingredients, batch, traceability, warnings or instructions.
Trading standards teams at Heathrow Airport and around London play an important role in protecting consumers from unscrupulous businesses seeking to bypass EU and UK safety laws.
There has been a surge in firms attempting to import sub-standard face masks, many with false labelling or faked safety certificates.
Trading standards teams are being pragmatic in seeking to let these important goods through, once misleading labelling is removed, and the necessary safety compliance can be shown.
However, we will continue to protect consumers from unsafe goods.
– STEPHEN KNIGHT, LONDON TRADING STANDARDS OPERATIONS