It's official: mergers and acquisitions can seriously damage shareholder value – if handled wrongly.

This is the main finding of new research by systems and technology analysts PA Consulting into different company approaches to post-merger integration.

The joint study into 85 mergers and acquisitions with Edinburgh University reveals the hidden cost of mis-matched deals. It said returns are highest in the case of mergers that are thoroughly planned and where the company taken over is left with some degree of autonomy.

John McGrath, of merged drinks group Diageo said: "There can be dangers in over integration, or in moving too fast in an attempt to realise all your synergies at once. You have to be selective when deciding exactly what to integrate, and how quickly."

Jeremy Stanyard of PA Consulting said: "The bigger returns to shareholders come from detailed advance planning, rigorous cash-based progress and from involving people from the target organisation as early as possible."