SFR has been criticised by the French regulator Arcep for "insufficient" mast sharing to rival Free Mobile.
Under regulations in the country, mobile operators must share passive sites where possible in order to maximise cost-effectiveness and reduce the environmental impact of network infrastructure.
Operators must allow "all reasonable requests" from rivals to access their sites, with Arcep handing down a special dispensation to Free because its late entry into the French market.
However, Free approached the regulator to resolve an issue it had with SFR. It claimed the rate SFR shared sites with it had slowed since the end of 2014 and that the operator had refused to deal with any concerns.
Arcep said the rate at which sites were being made available by SFR was "insufficient", adding that it failed to hit the regulatory obligations it was under.
In a statement, Arcep said: "To ensure that it provide Free Mobile with full and diligent access to its sites, Arcep has instructed SFR to offer Free Mobile, within a tight deadline, an amendment to the agreement in question, accepting the order from Free Mobile and setting out a timetable for making the sites available.
"SFR must therefore comply with a minimum rate of production, as defined by Arcep, provided that Free Mobile also comply with its contractual obligations."
Arcep said a full decision would be published in the near future. In the meantime, it has the task of handling this month's 700MHz auction.
The sale is due to begin next week.