Renee delays very, very bad signs of ageing with 'sandblasting facial'

But then, with an arduous beauty regime at her disposal - including a dosing of neat oxygen and some facial 'sand blasting' - it is little wonder that the 37-year-old actress looks so polished.

Indeed, before every glamorous premiere, the American star undergoes a bizarre biochemical facial routine, designed to rid her skin of any blemishes and keep her looking young.

As well as a £145 'triple oxygen treatment', Miss Zellweger endures a 60 minute 'microdermabuffing' session, described by practitioners as 'sand-blasting for the face'.

'Renee may have perfect skin now but it hasn't always been that way,' said a source close to the star.

'She discovered the triple oxygen facial a couple of years ago and has been a convert ever since.

'They are by no means the most relaxing of treatments - Renee has to lie supine for two hours while gas is sprayed onto her face and she is wrapped in cling film - but there is a lot of science behind them to suggest that they work. And, of course, Renee is convinced that they do.

'She really does have an even, baby-soft complexion nowadays and she looked absolutely amazing when she stepped out on the red carpet just 24 hours after her last treatment.'

Miss Zellweger had her last session at the Bliss spa in Chelsea, London, earlier this month - the day before she appeared, looking radiant, at the premiere of her latest movie, Miss Potter.

During her two hour triple oxygen sessions, the actress' face is cleansed and exfoliated before she is given a pre-extraction oxygen wrap to open up her pores.

Here, two sheets of cling film are wrapped around her forehead, chin and cheeks - leaving only her nose exposed - to help activate the oxygen and milk face mask underneath.

When the film is removed, Miss Zellweger's therapist spends around ten minutes squeezing out any blackheads or pustules.

From here, a vitaminised oxygen spray, containing vitamins A, C and E, is sprayed all over her face and neck from a hand-held canister.

Next up, Miss Zellweger is 'sand-blasted' with the help of a three foot long, two-pronged tank which delivers a stream of hard-hitting micronized crystal particles.

During the intensive, 45 minute resurfacing treatment, aluminium oxide crystals, which are feted for their anti-bacterial properties and for being second only to diamonds in hardness, are sprayed onto her face through a stainless steel wand.

The particles cut across the surface of her face, removing the top layer of epidermis.

Dead skin cells and the used crystals are then vacuumed back up into a waste container filter attached to the tank.

By breaking apart dead cell surface layers, microdermabuffing helps reduce the appearance of age spots and sun damage, as well as smoothing out the onset of fine lines and wrinkles.

While the process is not unbearably painful, it can sting, leaving the skin looking pink and raw.

Accordingly, the Bridget Jones star is given a post-abrasion regenerative oxygen spray to help soothe the face.

After her three hour spa session, Miss Zellweger is advised to avoid direct sunlight and asked to go without make-up for at least six hours.

While the benefits of such a rigorous beauty regime vary from person to person, oxygen is required by the body to keep the skin fully functioning.

It also kills surface bacteria - which is anaerobic and so cannot survive in the presence of oxygen - thus helping to keep skin clear and spot-free.

The amount of oxygen that reaches facial tissues via natural blood micro-circulation are also thought to drop by up to 70 per cent by the age of 30, meaning that regular, concentrated oxygen boosts help skin stay looking young.