Sir Ian McKellen has waded into the "gay red shirt" debacle, saying Prime Minister John Key needed to "watch his language".

Key landed himself in hot water after he reportedly said David Beckham was "thick" and the same day told a radio host his red jersey was "gay".

He has repeatedly refused to deny calling football legend Beckham "thick as bat sh**" during a visit to a Dunedin school last Friday.

McKellen, who has a strong association with New Zealand, said on his website the comments were "careless" and did nothing for New Zealand's reputation.

"New Zealand has an undeserved reputation (amongst those who have never visited) as living a little in the past, not quite up-to-date with the world elsewhere. As a regular visitor, I've often pointed out how, on social issues, the Kiwis have lead the world - first country to give women the vote, a nuclear-free stronghold and in the vanguard of civil rights for gay people.

"Which brings me to John Key, the prime minister of New Zealand. Recently in a light-hearted radio interview he referred to his host wearing a "gay red top", by which he meant, apparently, 'a weird red shirt'.

"Defending himself later, he said he was using the word in the sense that his children used it and not in any way to disparage gay people."

McKellen said Key should nevertheless "watch his language".

"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."

Key said he was serious when he needed to be but had to joke around sometimes.

However he stood by the comment that the host of a farming radio show would not do well in a charity golf event because he was wearing a "gay red top".

"You're munted mate. You're never going to make it. You've got that gay red top on."

Key said yesterday that he had meant "weird".

"As Prime Minister you have a wide range of functions and responsibilities that you undertake."

He said he was serious when dealing with the global financial crisis, the Pike River tragedy, the Rena grounding, and the Canterbury earthquakes.

"I dare you to show me one example where I haven't discharged my responsibility seriously, professionally and appropriately," he told TV3's Firstline.

"People have had plenty of chances to see me in action."

Then there would be times when he was being jocular and having fun.

"I'm often at events when they're quite light-hearted social events when people would want me to kid around."

That was the nature of being prime minister, he said.