Fashion designer-turned-director Tom Ford felt obliged to send an apology to Colin Firth after suggesting the British actor needed to slim down before stripping off for their new movie A Single Man.

Firth portrays a gay professor in Ford's directorial debut and sheds his clothes for one scene, in which his naked body is seen floating in water.

But Ford feared he had offended Firth during recent interviews to promote the movie when he candidly explained to the press how he'd advised the Bridget Jones' Diary actor to hire a personal trainer and get in shape before production began on the project.

The filmmaker admits he can't actually remember his exact words, but was horrified when he read his comments in the media.

So he decided to reach out to the star and say sorry in case he had hurt his feelings.

Ford says, "It's terrible. I emailed Colin about this... because you have to be so careful what you say to the press. The email to Colin said, 'Sometimes in life I can be a little overly blunt.' So I probably said (to the press), 'Colin, you're fat' - but I'm not sure I actually said that! I don't know and now people have picked up on it!"

But Firth admits he didn't take offence: "He (Ford) tended to handle me in 'honey' terms like, 'You look really great as you are, but if you want to get a trainer to come to your house every day I'll pay for it and it's fine.' That means you're fat!"

Firth was glad of the encouragement: "I guess he did me a favour, giving me one final push in the war against gravity. I'm in my late 40s so it was probably quite helpful."

The actor didn't hit the gym however, telling WENN, "I just ate less."