The owner of Cardiff, Luton and Belfast International airports has agreed to a takeover.

Abertis, one of Europe's leading infrastructure groups, which is the main party behind the 551.3m approach, manages more than 1,500 kilometres (937 miles) of toll roads but wants to build up its airport interests - currently based around the operation of two runways in Colombia.

TBI, who own the airports, said the tie-up gave it the financial strength to accelerate the development of its airports.

Luton Airport is already the subject of a 30 million redevelopment as TBI bids to meet increased demand from low-cost airlines easyJet and Ryanair.

The overhaul will provide Luton with room for 12 million passengers a year, although TBI is also in talks about possible further expansion at the site.

TBI chief executive Keith Brooks said of today's agreement: "Employees will be joining an enlarged group with international scale and financial strength ideally placed to accelerate the development of TBI's airport assets."

The bid has been tabled by Airport Concessions and Development, which is 90% owned by the Madrid-listed company Abertis, and 10% by Aena, which is one of the world's largest airport operators.

The terms of the offer - representing a 22% jump on TBI's closing price prior to news of a possible bid last week - have been backed by the company's board but still require the support of TBI's shareholders.

The deal provides a windfall for TBI chairman Stanley Thomas, who is thought to own about half of his family's near-20% holding in the company.

He was previously managing director of Peter's Savoury Products, a substantial food manufacturing business sold to Grand Metropolitan in 1988.

Mr Thomas was at the helm as TBI - formerly a South Wales property business - moved into airports and joined the London Stock Exchange in 1994.