Veteran actor Sir Ian McKellen has criticised New Zealand Prime Minister John Key, accusing him of using the word “gay” in a “careless” way.

Sir Ian, who plays the wizard Gandalf in the forthcoming Hobbit films, warned Mr Key to “watch his language”.

The Hobbit and the Lord of the Rings movies are filmed in New Zealand and are a major source of income for the country.

Mr Key made the remarks last week in a radio interview, in which he mocked the host for wearing a red jumper.

In an attempt to defend his behaviour, Mr Key said: “gay” was “just a slang term” used by young people and found in the Oxford dictionary.

“If someone was offended by it then I apologise but it’s not exactly like a term you don’t hear everywhere,” Mr Key said.

However, Sir Ian, who continues to be a tireless LGBT campaigner, wrote on his blog:

“Recently in a light-hearted radio interview [Mr Key] referred to his host wearing a ‘gay red top’, by which he meant, apparently, ‘a weird red shirt’.

“I’m currently touring secondary schools in UK, attacking homophobia in the playground and discouraging kids from the careless use of ‘gay’ which might make their gay friends (and teachers) feel less about themselves.

“So even as he supports the proposal to introduce same-gender marriages in New Zealand, I do hope John Key listens to his critics and appreciates their concern. Careless talk damages lives.”