The Czech billionaire circling Royal Mail is “very serious” about a takeover, a source close to negotiations has said, as the postal service prepares to mount a defence. Daniel Kretinsky, the investor known as the “Czech sphinx”, is expected to table an improved offer for Royal Mail’s parent company International Distribution Services (IDS) after his initial £3.1bn bid was rebuffed. The board of IDS is understood to be bracing for a fresh approach and preparing a defence. A source said Mr Kretinsky’s initial approach to the 500-year-old postal service had been interpreted as an effort to drum up interest from other shareholders. He is expected to make a fresh swoop before a deadline of May 15 imposed under City takeover rules. Royal Mail will meet investors next week to discuss the approach, which it has branded “opportunistic”. One shareholder described Mr Kretinsky’s 320-per-share bid as “insulting”, telling the Financial Times: “The offer is an absolute joke and I suspect he knows that.” Another major shareholder said it had prepared a document outlining its views, but was told by Royal Mail not to go public until after the meeting. Red Wheel, one of IDS’s largest investors, has previously signalled it would...