The show will be his first TV appearance since cancelling his highly anticipated live tour in December due to “unforseen family circumstances”.
At the time, the 44-year-old comic said his “family must always come first” and that the decision to cancel his first live tour in eight years was “not taken lightly”.
Car Share tells the story of a supermarket assistant manager John Redmond (Kay) and promo rep Kayleigh Kitson (Sian Gibson) who share a lift to work.
Kay said he wanted to try the unscripted episode because it is “something original” and he “couldn’t think of any other narrative comedies that have ever made a fully improvised episode”.
He added: “As Sian and myself often ad-lib when filming, we thought we’d give it a go for an entire episode.”
The second series of Car Share ended on an emotional cliff-hanger, with Kay’s character rejecting the advances of his colleague Kayleigh.
Ahead of their TV premiere, the final two episodes of the show were shown to a packed audience at a charity screening at the Blackpool Opera House last month.
Raising money for The Lily Foundation – a charity which supports children with mitochondrial disease – Kay made a personal appearance and gave a short speech ahead of the screening.
The unscripted special was described by one audience member as “pure genius”.
The special edition of Car Share will air at 10pm tonight, with the final episode following three weeks later on the second bank holiday of the month on 28 May.
Kay – who has been named the most successful comedian of all time by Guinness World Records – has confirmed there will be no third series, telling fans he preferred to “go out on top”.