Ofcom is to investigate Channel 4 after an extended outage of its subtitle services last year.

The broadcaster experienced a number of major problems from September, caused by issues at the centre which handles its playout services.

Its subtitling, signing and audio descriptions were among those affected – prompting the National Deaf Children’s Society (NDCS) to call for regulatory action to be taken.

Ofcom said Channel 4 had fallen short of its subtitling quota on Freesat, a subscription-free satellite TV service offering more than 170 TV and radio channels.

A statement from the media watchdog 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.

“However, Channel 4 fell short of its subtitling quota on Freesat, a satellite TV platform used by around two million UK homes.”

The issues arose in September after a fire suppression system was triggered at the broadcast centre of Red Bee Media, severely damaging a large number of hard disks in a variety of systems.

The “extended outage” of Channel 4’s broadcast channels was not fully resolved until November 19.

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”.

It added: “So, as well as investigating Channel 4, Ofcom is undertaking a review of the transmission arrangements and back-up facilities the affected broadcasters had in place at the time of the outage, and what changes they have made or planned as a result.”

It said 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”.

The broadcasting watchdog had previously criticised Channel 4 for not having “strong back-up measures”.

A statement from Channel 4 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.”

Red Bee Media said: “Following the incident, we collaborated with our customers, landlord and business partners, and all services are now restored. We have also undertaken extensive measures to understand the cause of the incident and are evaluating all options to make sure it doesn’t happen again.

“The services most affected were operating on older technology platforms. 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.

“Multiple customers are in the process of transferring to the new platform, and it is our ambition that all customers will do so. All new customers will also automatically be on the new platform.”

The NDCS previously called on Ofcom to intervene and resolve the “completely unacceptable” delay in fixing the issues.

Responding to the investigation, Ian Noon, head of policy at the NDCS, said: “During the prolonged period when these services were unavailable, many deaf young people were left upset, frustrated and angry, not least because they no longer felt included or provided for.

“Information on when they would be resumed was also too scarce, leading to a lot of uncertainty among the thousands that rely on them.

“It’s important that these issues are investigated by Ofcom when broadcasters don’t deliver. Unless lessons are learnt, deaf young people risk being failed again.”