Tompkinson is best known for TV credits including Drop the Dead Donkey, Ballykissangel and DCI Banks. His stage appearances include Art (West End) and The Revenger's Tragedy (Royal Exchange).
He'll be joined in the new cast by Anna-Jane Casey (Cats, Starlight Express, West Side Story, Chicago) as The Lady of The Lake, while former EastEnder Todd Carty will continue in the role of Patsy.
"Lovingly ripped off" from the 1975 film Monty Python and the Holy Grail, Spamalot tells the tale of King Arthur and the Knights of the Round Table in their quest to find the religious relic – and features a chorus line of dancing divas (with serfs), flatulent Frenchmen, killer rabbits and a legless knight.
The musical has a book and lyrics by original Python Eric Idle, who also co-wrote the music with John Du Prez. The West End production, which ran at the Palace from 16 October 2006 (previews from 30 September) to 3 January 2009, reunited the Broadway creative team including director Mike Nichols and set and costume designer Tim Hatley.
First seen in 2005, Spamalot returned to the West End earlier this year with an eight-week run at the Harold Pinter Theatre starring John Culshaw and Marcus Brigstocke. It's currently booking at the Playhouse until 13 April 2013.