Helen Mirren follows a military-style fitness regime.

The 'Calendar Girls' actress has revealed the secret to keeping in shape aged 68 is sticking to the Royal Canadian Air Force's exercise plan which was devised by a doctor in the 1950s.

Helen insists the routine - which takes just 12 minutes and includes push-ups, sit-ups, running on the spot and arm circling to work all the muscle groups - helps her to get fit in just a couple of weeks.

She told Hello! magazine: ''I do a thing that leads me into exercise. It is the Royal Canadian Air Force exercise plan.

''It is 12 minutes and they have charts you follow... Each day, you have to do the exercises within the 12 minutes and until you can, you can't move up.

''It is the exercise I have done off and on my whole life. It just very gently gets you fit. Two weeks of doing that and you think, 'Yeah, I could go to the gym now'.''

Helen - who is a self-confessed 'yo-yo dieter' - revealed that her weight constantly fluctuates by 10lb and is returning to the plan after finding herself at the upper end of her ideal weight range.

She explained: (I'm) at the top end of those 10lb and I have got to get back to the bottom end'.

Despite her intense fitness regime, the Oscar winner also admitted that she no longer feels comfortable wearing a bikini.

She said: ''I am beyond the bikini-wearing age, really. I wouldn't normally wear one. I've always looked awful in a bikini, even when I was young.''