The Big Bang Theory: Picture: Supplied

A spin-off of the CBS sitcom 'The Big Bang Theory' has officially been given the green light.

Speculation about a programme following Sheldon Cooper's early life has been circulating for months, and US television network CBS has now officially confirmed that a new show will centre on the fictional theoretical physicist's days at high school in Texas.

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A full series of episodes is poised to air during the 2017-18 TV season and will see Iain Armitage star as the young Sheldon, whose older incarnation has previously been played by Jim Parsons.

Jim, 43, won't appear on screen during the series but has been handed the responsibility of narrating the programme.

It's been announced, too, that none of Sheldon's friends in adulthood – Leonard Hofstadter, Howard Wolowitz, Rajesh Koothrappali and Penny – will feature in the spin-off.

Iain Armitage at the Los Angeles premiere of the HBO series, "Big Little Lies." Armitage will star in “The Big Bang Theory” spinoff, "Young Sheldon," portraying Jim Parson's character Sheldon Cooper at the age of 9. Picture: Jordan Strauss/Invision/AP

Meanwhile, Kunal Nayyar – who has played Rajesh in the US sitcom since it began in 2007 – predicted in 2016 that 'The Big Bang Theory' will end in the next five years.

Kunal wants to portray Rajesh for as long as possible, but he admitted the show realistically only has few years left.

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Speaking exclusively to BANG Showbiz, he said: "We have one more series in our contract, that's Season 10. Other than that I don't know. But I'll keep doing it for as long as they will have me.

"We could all have walking sticks doing 'The Big Bang Theory' Season 20. I think realistically there's less time left now than there was on the show before."

The show is currently in its tenth season – and Kunal admitted he is unsure when it will end.

He shared: "Maybe it will go to 11, 12 or 13. I will do it as long as I can. But I think it is somewhere in that range."