It was an unpleasant sight to see a sickly Dame Maggie Smith being shuffled into the back of an ambulance on Tuesday.

Thankfully, she was busy shooting a dramatic scene on the set of her new BBC film, Lady in the Van, based on Alan Bennett’s autobiographical West End play that goes by the same name.

The 79-year-old actress has swapped her Downton Abbey finery for a less glamorous look as she embodies the character of eccentric homeless pensioner Miss Shepherd in the movie.

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Based on true events, for 15 years Miss Shepherd lived in a battered old yellow van parked on Bennett’s driveway in Camden, North London.

The playwright, who wrote about his experience and relationship with Miss Shepard in his diaries before sharing the story on stage, is played by Alex Jennings.

The flick, written by Bennett and directed by Nicholas Hytner, is their second collaboration since 2006’s The History Boys.

A few weeks back, the iconic film’s breakout star James Corden was also spotted on the set filming a cameo role as a market stall vendor.

The almost unrecognisable Dame truly looked like a tramp in worn-out, frumpy layers of clothing.

Her greasy grey hair was ratty and unkempt and the luxury of make-up was forbidden.

A bright red blanket covered her lap as she was manoeuvred into the back of an ambulance in a wheel chair.

Her appearance was enough to send a shiver down Violet Crawley’s spine.

But it was all in the name of art and there are already high expectations for the film when it is released next year.

Bennett described Miss Shepherd in his diaries as: ‘Nearly six foot, she was a commanding figure and would have been more so had she not been kitted out in greasy raincoat, orange skirt, Ben Hogan golfing cap and carpet slippers.’

After watching her suffer at the hands of children and drunken revellers, the playwright took pity on her and agreed to let her park her vehicle on his driveway – but he wasn’t happy about it.

Detailing his experience, he admitted: ‘It made me furious that I had been driven to such a pass. But I wanted a quiet life as much as, and possibly more than, she did. In the garden she was at least out of harm’s way.’

However, when she passed away in 1989, Bennett discovered that there was more to Miss Shepherd than he realised.

His ‘tenant’ had actually been a talented pianist who suffered a series of mental breakdowns.