Chief executive Bill Sweeney says they had budgeted to make a loss before the coronavirus outbreak, but it will now be considerably more as the virus causes havoc across the world.
The closure of Twickenham will have hit revenues particularly hard, while the RFU have also handed out £7million in loans to help struggling community clubs during these hard times.
Sweeney issued a statement on Wednesday, part of which read: "When we do not stage matches and events we do not generate revenue.
"Based on our planning assumption we estimate RFU revenue losses over the next 18 months to be approximately £45-50m and have a firm plan in place to mitigate this."
Part of the plan will see Sweeney and other executives taking a pay cut of more than 25%, while it has also been suggested England coach Eddie Jones and his staff could be asked to take a reduction in salary.
Meanwhile, Sweeney says an update on the rest of the 2019/20 season being played out, will be given by mid-April.
"We are managing in the unknown," he added. "We have modelled three potential scenarios and are working on an assumption based on a medium-term impact with a view to a return to rugby in the autumn.
"We will continue to monitor against this assumption and review and revise planning where necessary."