The owner of Westfield shopping centres has agreed to a $24.7bn (£18.5bn) takeover offer from French property giant Unibail-Rodamco.

The offer values Westfield shares at $7.55, which represents a 17.8% premium to their value at the end of Monday's trading session, and the two companies said they expect the takeover to be completed by mid-2018.

Paris-based Unibail-Rodamco is the largest commercial landlord in Europe and operates 71 sites across the continent, while Westfield has giant malls in London's White City and Stratford.

Sir Frank Lowy, who co-founded Westfield in the 1960s, described the deal as the culmination of a strategic plan that the Australian company launched three years ago, when its Australian and New Zealand assets were separated from those in the UK and US.

"Unibail-Rodamco's track record makes it the natural home for the legacy of Westfield's brand and business," he said. "We look forward to seeing Westfield continue to grow as part of the world's premier owner of flagship shopping destinations."

The deal, which values the stake of the Lowy family in the Australian behemoth at just under $2bn, will see Lowy step down from his role as chairman, although his sons Peter and Steven will remain prominent figures.

The newly-merged company will control over $72bn worth of assets across Britain, Australia, the US, Canada, New Zealand and France and the takeover is expected to result in $100m worth of cost savings per year.

"The acquisition of Westfield is a natural extension of Unibail-Rodamco's strategy of concentration, differentiation and innovation," said Unibail chief executive Christophe Cuvillier.

"It adds a number of new attractive retail markets in London and the wealthiest catchment areas in the United States. It provides a unique platform of superior quality shopping destinations supported by experienced professionals of both Unibail-Rodamco and Westfield."

News of the merger comes just a week after Hammerson, the owner of the Bullring shopping centre in Birmingham, reached an agreement for the takeover of rival Intu for approximately £3.4bn.

The deal will create Britain's biggest property company, which will have combined assets worth £21bn across the UK and Europe and a stake in 12 of Britain's 20 so-called supermalls - large shopping centres of more than 20 million sq ft that attract more than 20 million customers a year.