Protests have been held in London and Sussex following the award of the new Thameslink franchise to Govia.
A joint venture between French company Keolis, which is majority owned by the French state railway, and Go Ahead, it has secured the Thameslink, Southern and Great Northern railways franchise.
According to the Trades Union Congress, Govia and Go Ahead will be allowed to reduce the number of guards on trains, introduce driver-only vehicles and cut station staff. The Action for Rail Campaign claims this could lead to safety compromises on the network.
Moreover, the group also claims profits made from the franchise will be reinvested in France rather than devoted to improving services in the UK.
A spokeswoman for Govia told the BBC: "We never compromise on safety. The safety of passengers and staff is always of paramount importance."
The Department for Transport branded the statements by rail unions "irresponsible scaremongering".