The venture, which will launch in mid-September, will take the name Large Corp. Large's management team will be comprised of Spectre's existing managing director, Bertie Miller, and Stark's joint managing directors, Stephen Gash and Cathy Green.
The partnership also brings together Stark Films' much-respected director, Jeff Stark, and Spectre's highly awarded Daniel Kleinman. They are joined by Spectre's Anthony Eston, John Lloyd and Enda McCallion, as well as Stark's Jon Greenhalgh, Colin Gregg and Dominic Savage.
Miller said he had been looking to move Spectre on to the next level after it was voted the Production Company of the Year at the British Television Advertising Awards 2003.
He said: "A boutique-style shop is lovely but it has to mature after a while. From day one, Large will be the biggest UK production company in terms of turnover.
"Large will offer strength and depth of talent and an experienced, fully staffed production department that will provide greater service to TV departments."
Stark Films was founded in 1991, while Spectre was established just five years ago. Kleinman said: "Large gives us both a broader base of talent.
Stark has a history and is a solid operation, while Spectre is relatively new and a bit edgier. Personally, it offers me the opportunity to be part of a strong group of people who will provide buzz and creative reference."
Kleinman's recent work includes the TBWA\London John Smith's campaign that stars the comedian Peter Kay, as well as M&C Saatchi's Foster's campaign.
Meanwhile, Jeff Stark directed Ogilvy & Mather's latest spot for the Ford Mondeo featuring the cartoon characters Tom and Jerry.
Gash said: "The move is a forward-thinking bid to offer both quality and volume.
"Both Stark and Spectre have very similar cultures. They both hold reputations for having a friendly and personal approach to production and Large will continue to hold these values dear.
"In short, Large has the strength to the power of two."
The possibility of uniting Stark and Spectre first came about as the result of a conversation between Miller and Gash earlier this year, when both men expressed their desire to take part in a merger.
Although advertising will remain its core business, Large will offer directors the opportunity to shoot promos and short films.
Large will be based temporarily in Beak Street while the company searches for more suitable offices.