Merlin Entertainments, the company that owns the National SeaLife Centre in Brindleyplace, Birmingham is merging with Legoland, which owns and runs four Legoland Parks, based on the Danish toy brick system.
Privately-owned Lego put Legoland up for sale last year to concentrate on its core toy products in the face of fierce competition.
Blackstone Capital Partners, the American venture capital company that acquired Merlin last month, is buying Legoland for 375 million euros (255 million) and merging it with Merlin.
Blackstone and managers own 70 per cent of the enlarged Merlin, while the other 30 per cent is held by Lego and the Danish Kirkbi A/S.
The deal adds Legoland sites in Germany, Denmark and California, as well as the big visitor attraction near Windsor, to 28 Merlin " SeaLife", "Dungeon" and "Earth Explorer" centres in eight European countries. Merlin's chief executive, Nick Varney, said the addition of the Legoland parks would make the company more attractive when it is floated on the stock market in about three years' time.
The merger, he added, creates "a balanced portfolio of 'chainable' brands which are transferable across borders and which is naturally protected against external factors such as weather, international visitor trends and local market economics".
Lego's chief executive, Jorgen Vig Knudstorp said the merger would create a "weather hedge, with the mainly outdoor Legolands complementing Merlin's indoor parks.
"Though the years I've listened to our park managers' excuses about the weather and now there won't be any more because we will have plenty of indoor activities through Merlin," he said.
It is thought that Blackstone fought off strong competition for the sites, which are being sold to free up cash for the Lego toy business.
The new company should be Europe's second largest visitor attraction business after Tussauds with 12 million visitors.
Lego's owner Mr Kjeld Kirk Kristiansen said it was important that Legoland found a new owner prepared to preserve the brand's values "of playful learning and creative experiences of a high quality".
Legoland's four sites attracted more than five million visitors last year, while 6.3 million people attended sites run by Merlin.
Lego, the world's fourth largest toy manufacturer, recently announced annual losses as it faced up to a weakening economy and the impact of growth in consumer electronics.
Mr Kristiansen said "The toy market has been declining for some time and is extremely competitive.
"To consolidate its position, the Lego Group chose to sell its majority holding in the Legoland parks in order to focus on closer relations with our customers and consumers, improved marketing and a greater emphasis on core products."
The news coincided with the first full-year results from the rival Center Parcs since it brought is shares to the stock market.
There was no direct comparison the year before with the profit of £14.8 million. But leaving aside tax, exceptional items and goodwill amortisation, Center Parcs made£26 million, up from £24.4 million in 2003.04. Sales edged up to £229.6 million from £227.7 million.
A final dividend of 2.37p makes the total pay-out for the year 3.57p.