The play, written by Alan Bennett, is an autobiographical piece about an elderly lady who took refuge in the author's Camden garden and has been a hit in the West End, reported Digital Spy.
TriStar Productions will partner with BBC Films for the movie, which will be written by Bennett and directed by Nicholas Hytner. The pair previously worked together on 'The History Boys'.
The 79-year-old 'Downton Abbey' actress will star as eccentric pensioner Miss Shepherd, who lived on Bennett's driveway for 15 years.
"My collaboration with Alan Bennett has been at the centre of my creative life and this true story is perhaps his most compelling and remarkable," Hytner said.
"Maggie Smith and Alex Jennings are two of Britain's greatest actors, and they...Are old colleagues. And it gives me particular personal pleasure to be working again with Tom Rothman (TriStar Productions Chairman0, my longest standing friend in the American film industry," he added.