OFT Chief Executive and Decision Maker Clive Maxwell said: "Our review found that these two trusts compete with each other in a number of specialities, both to attract patients and funding from commissioners.
""We decided on the basis of evdience available that we could not rely on the potential benefits from this merger outweighing the expected impact on patients and commissioners of a loss of competition.
"We have therefore concluded that a reference to the Competition Commission for an in-depth review is necessary."
Commenting on the referral, Tony Spotswood, proposed cheif executive of the merged organisation, said: "We are pleased that we are one step further in this delicate process.
"There will now be a period of further analysis, which we anticipated, being the first Foundation Trusts in the UK to follow this process. It will hopefully reassure us all that a merger is indeed in the best interests of patients and staff."