Mark Millar has teased that The Secret Service would be Matthew Vaughn's "best movie yet".
Sir Michael Caine and Colin Firth have signed onto the big-screen adaptation of Mark's graphic novel, with Adele, David Beckham, Sir Elton John, Samuel L Jackson, Tom Cruise and Leonardo DiCaprio all reported to have been offered a part in the spy film.
"I think this is going to be Vaughn's best movie yet. This is the fifth movie he's directing and the screenplay of this ties with Pulp Fiction as my favourite screenplay of all time. It's just so good," said the Scottish writer.
Mark remained tight-lipped over the list of stars rumoured to make an appearance, but said he is thrilled about Michael and Colin in the lead roles.
Colin is set to portray "the world's greatest secret agent", who is training his nephew - played by newcomer Taron Egerton - in the spy business, with Sir Michael cast as the head of the elite spy organisation.
"The fact that Michael Caine's in it is just nuts. Having him in one of your films is a dream come true. And Colin Firth! All the girls in the family are going nuts for that," said Mark.
The Secret Service, scheduled to start filming in the autumn for a 2014 release, would mark the fourth reunion between Matthew and screenwriter Jane Goldman, following their collaboration on Stardust, X-Men: First Class and Kick-Ass.
"Matthew Vaughn and Jane Goldman, they are the winning team. To have them on Secret Service is great," Mark continued.
"We did the comic a couple of years back and it's in such safe hands now. Matthew's a great director to work with when the books are being adapted because he keeps you so involved. It's really exciting seeing it all coming together."