However, they also confirmed that future diversions could only be set up overnight because of the traffic disruption it would cause during daytime.
Mid Scotland and Fife Conservative MSP Murdo Fraser said that would infuriate drivers.
An experimental closure of both bridges was carried out last month to test the diversion plans.
The Queensferry Crossing had to be shut twice last year because of the threat to vehicles from ice falling from its towers and cables.
That followed The Scotsman revealing several windscreens were damaged by falling ice in 2019.
Sensors have been installed to warn of the problem and research is ongoing into finding a way of preventing ice posing a risk.
It was not thought likely to be a major problem when the bridge was designed, with only one such incident happening on the 56-year-old Forth Road Bridge.
Officials said such "ice accretion” on the Queensferry Crossing required a specific combination of weather conditions - when temperatures of around 0C were accompanied by particular levels of relative humidity, specific wind speeds and wind direction and coincided with the dew point – when air becomes saturated with water vapour.
The Forth Road Bridge is normally reserved for buses and taxis.
It could not be used for other traffic during the last ice closure last month because of work to replace its carriageway expansion joints.
Mr Fraser said: “This revelation is utterly shocking and will infuriate all those who had understood that it would be available at fairly short notice.
“Surely the whole point in using the Forth Road Bridge as a diversion was to alleviate congestion and chaos for motorists if the Queensferry Crossing has to close again?”
“It is a very strange way of trying to solve the problem of the Queensferry Crossing closing and is not a reactive measure.”
Philip Gomm, of the RAC Foundation motoring group, said: "The real question is why only now is the Forth Road Bridge being considered as an emergency diversion?
"Shouldn't the benefits and limitations - including how to put out cones - have been analysed years ago?
"The current problem might be ice, but like any such critical piece of infrastructure, the Queensferry Crossing will always be susceptible to things like terrorist attacks, and fuel and chemical spills, where an adequate diversion route is essential."
Chris Tracey, bridges manager for roads operator BEAR Scotland’s south east unit, said: “An emergency diversion would only be implemented overnight, outside peak periods.
“This is because it takes several hours to remove roadside barriers and lay large numbers of cones and signs.
"This process would cause significant additional disruption to traffic during busy periods.
"A trial diversion over the Forth Road Bridge will be carried out in January, with the aim of further refining this process.
A BEAR Scotland spokesperson added: “The test carried out last month showed we could comfortably implement a diversion over the Forth Road Bridge overnight, before the morning peak.”