Broadcasting watchdog Ofcom is to investigate Channel 4 after an extended outage over its subtitle services.
The broadcaster experienced several major outages late last year, caused by issues at the centre which handles its playout services.
Its subtitling, signing and audio descriptions were all affected, prompting the National Deaf Children's Society to call for action.
Channel 4 told the BBC it was sorry for "significant impact" of the outage.
A statement from Ofcom said: "We have found that Channel 4 managed to meet the statutory requirement to subtitle 90% of its programme hours over 2021 on most programmes.
"It also met its requirements for audio description and signing."
But the media watchdog added: "However, Channel 4 fell short of its subtitling quota on Freesat, a satellite TV platform used by around two million UK homes."
The issues originally arose in September after a fire suppression system was triggered at the broadcast centre of Red Bee Media, causing severe damage.
The "extended outage" of Channel 4's broadcast channels was not fully resolved until November.
In a statement to BBC News on Friday, a Channel 4 spokeswoman said: "We apologise for the significant impact the Red Bee Media incident had on our access services.
"Channel 4 would like to reassure our audiences that we have thoroughly reviewed the resilience of our systems to ensure that such a catastrophic event cannot harm our ability to deliver these essential services in the future.
"Whilst we have not met our own high standards in 2021, we still delivered all of our overall statutory obligations and we are once again offering market-leading access services," she added.
Ofcom "remains very concerned" about the incident, which it said resulted in a "lengthy outage to Channel 4's access services provision and also wider disruption to its general broadcasts on all platforms".
The authority is now reviewing Channel 4's transmission arrangements and back-up facilities that were in the place at the time, and what they have done about it since.
It noted how the broadcaster's "access services" are relied on "by millions, including deaf, hard-of-hearing, blind and partially-sighted people, to watch and listen to television".
In a statement Red Bee Media said "all services are now restored" and that it had taken "extensive measures to understand the cause of the incident". The company continued by saying it was now "evaluating all options to make sure it doesn't happen again".
"The services most affected were operating on older technology platforms," it said. "Red Bee's new platform is inherently more resilient and customers who were already on this platform experienced minimal disruption as a result of the incident."
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