"Either merger would make it more difficult for other exchanges to compete with LSE in trading UK equities due to both bidders direct control or influence over the provision of clearing services, " it added.

BBC: The watchdog's report could spark further bids for the LSE

The UK clearing house would advise the consumer about making a claim, which could be sent to the clearing house in the other country or direct to the dispute resolution body.

BBC: Europe e-commerce initiative