(Adds comments on acquisition, investments, Royal Mail details; paragraphs 4-10)

PRAGUE, May 29 (Reuters) - Royal Mail and its international parcels network GLS need sizable, almost immediate investments to defend market share and face shifting market trends, Czech billionaire Daniel Kretinsky told Reuters on Wednesday after the group's owner agreed to a 3.57 billion pound ($4.55 billion) takeover.

"It is important for logistics companies not to miss this out-of-home delivery wave, which means they need to be ready to invest now," Kretinsky said.

"We believe that if the group doesn't respond properly on the out of the home solutions it may have a detrimental impact on its market share. And specifically in the UK, any shrinkage of the market share would be fatal."

The offer valued International Distributions Services , owner of Royal Mail and GLS, at 370 pence per share.

Kretinsky, IDS' biggest shareholder with a 27.6% stake, said the acquisition fit with his EP Group's strategy aimed at logistics as one of its core focus areas.

Royal Mail, whose iconic red post boxes with the Royal crest dot the country, has struggled with labour strikes, competition and loss of market share.

Kretinsky, in a rare interview done online from a car while in Paris, said that taking over the company would allow management to focus on longer-term strategy better than as a public company.

He said about 20,000 parcel boxes for package deliveries in Britain would require investment of 350-400 million pounds ($508.44 million) for Royal mail, and GLS needed about the same.

"We will encourage the management, give permission to management to implement very quickly such a rollout," he said.

It could be financed from internal sources and, if needed, Kretinsky's parent group's free cash flow generation of 2-3 billion euros per year, he said. ($1 = 0.7867 pounds) (Reporting by Jan Lopatka, Jason Hovet and Anousha Sakoui. Editing by Jane Merriman)