The outgoing chief executive of the Association of Private Client Investment Managers and Stockbrokers, a powerful UK trade body, has warned the European Commission that Deutsche Börse's proposed merger with Euronext will harm competition and could lead to "substantial cost issues".

Angela Knight, who is set to leave as head of Apcims next April to become chief executive of the UK British Bankers' Association, flagged concerns from the trade body's members in a letter to the European Commission after it was invited to participate in an enquiry into the planned takeover.