His friend, actor Toby Hadoke, confirmed that Kay was found dead at his north London home on Monday.
The cause of death has yet to be determined.
He wrote on Twitter: “It is worth great regret that I announce the sad death of the actor Bernard Kay who passed away over Christmas aged 86.
"RIP Bernard.”
He later added: “Admin done, so now can I just say that Bernard Kay was one of TV's finest ever character actors: always nuanced, understated & intelligent. (sic).”
Hadoke made sure to pay tribute to his talented friend, writing: "He was one of those superb understated but versatile actors that we don't seem to have any more.
"He never gave a bad performance.
"He was greatly admired by his peers.
"His sense of humour was combative but there wasn't any meanness in him."
Bernard Kay was married to Patricia Haines, previously the wife of Michael Caine, from 1963 until her death in 1977.
The actor had an extremely wide-ranging TV career - starring in Foyle's War and The Crusade as well as appearing as Clive Phillips in Coronation Street, and bagging a role in Doctor Who.
He also appeared on other TV programmes such as Emmerdale, Doctors and Casualty.
Our thoughts are with his friends and family at this sad time.