The Competition and Mergers Authority said it would now explore whether the £271m deal to create the UK’s largest FM business would reduce competition.
Under the proposals, both company boards hoped to sign off the deal by the end of this year.
“The transaction was voluntarily referred to the CMA for clearance and we are working closely with the CMA to progress the process,” said a spokesperson for Mitie.
The CMA is inviting interested parties to offer their comments before 29 September.
Interserve would receive £120m in cash and a 23.4% shareholding in Mitie, which would then employ 77,500 staff in both hard and soft FM services.
Last year Interserve Facilities Management achieved revenue of £1.4bn, generating operating profits of £43m.
Mitie, which presently employs 37,500 staff, delivered an operating profits of £86m from £2.2bn turnover in the year to March.
Email CMA about any competition issues.