The deal, in which Cook paid Spain's Grupo Viajes Iberia an undisclosed sum, bolsters the agent's presence in the UK tour industry which is dominated by Canada's Thomson, and British rivals Airtours and First Choice.
Sunworld controls 5 per cent of the British short-haul package holiday market behind the three largest players, which have almost 60 per cent between them, and Cosmos/Avro with 6 per cent.
"We have always enjoyed an excellent relationship with the company and today's announcement is a natural extension of our existing association," Ulrich Zierke, the chief executive, said. Sunworld's brands include Summersun and Wintersun, Sunworld Sailing, Sunworld Beach Villas and the charter airline Airworld.
Thomas Cook said Sunworld would complement its travel agency operations, long-haul holidays and foreign-exchange business. It has installed Nigel Hards, its former director of UK retail operations, as managing director, replacing chief executive Peter Long who is leaving immediately.
The deal with Sunworld draws a line under Thomas Cook's attempt to form an alliance with First Choice, in which it took a 21 per cent stake in 1993 as part of the tour operator's successful defence against a hostile bid from Airtours.
The partnership, designed to counter the vertically integrated operations of Thomson, which owns travel agent Lunn Poly, and Airtours, which owns Going Places, never lived up to its potential and the relationship has been on the rocks for a while.
A spokeswoman for Thomas Cook refused to say whether the Sunworld deal would mean it would sell its First Choice stake but she did say that it would have no impact on the commercial relationship between the two companies.
"We will continue to sell their products through our shops," she said.
The deal signals a significant shake-up in the UK holiday industry at a time when all the large players are reviewing their strategies to counter the cut-throat competition and over-capacity that blew a hole in profits last year.
Airtours has been expanding rapidly in North America and Scandinavia. Thomson, meanwhile, is denying rumours that it is planning to sell or float its UK Thomson Travel arm.
Separately yesterday, Inspirations, Britain's sixth-largest tour operator, said it was paying pounds 1.75m to acquire Skiers World, a ski-tour group. The deal will be funded by a pounds 12m rights issue of two convertible shares for every five ordinaries.