Up to 600 heavy goods vehicles (HGVs) will be permitted, northbound only, between 11pm and 4am every night, weather permitting.
However, the bridge will not now be fully re-opened to vehicles over 7.5 tonnes until mid-March because storms over the last few weeks have held up repairs, and more damage has been found.
Other traffic was allowed back on the crossing on 23 December after it was closed for nearly three weeks following a fault being found with the “truss” structure which supports the southbound carriageway.
The lorry ban was expected to be lifted in mid-February, but this has now been postponed until mid-March.
Transport minister Derek Mackay said a second phase of repairs had to be completed before lorries can be given unrestricted access.
The impact of lorries crossing the bridge would be monitored to see if their number could be increased.
“Preventative works” are also required at two other locations on the truss.
Mr Mackay said: “Allowing limited access to the bridge when traffic is lighter will hopefully provide some relief to local hauliers while repair work continues.”
The Liberal Democrats and hauliers called for compensation for businesses affected.
The Freight Transport Association (FTA) also urged the relaxation of drivers’ hours being extended.
The Road Haulage Association (RHA) expressed concern at the limited number of lorries allowed to cross and said allowing them full access was “a matter of extreme urgency”.
Scottish Liberal Democrat leader Willie Rennie said: “This news will come as a bitter blow to businesses which are already struggling as a result of the closure of the bridge, and to the communities in Fife who have experienced huge disruption as a result of the spike in heavy goods vehicles forced to find alternative routes.
“Ministers need to devise and publicise a compensation scheme for the businesses affected.”
Lorries crossing northbound will have to queue, with one allowed to cross every 30 seconds to minimise potential damage to the bridge.
Some 7,000 lorries a day normally cross the bridge.
A spokesman for the Scottish Government’s Transport Scotland agency, which owns the bridge, said: “Relaxing the current HGV restrictions during the night when traffic volumes are lighter will help to mitigate the impact of the closure, without causing over stressing of the structure.
“A dedicated HGV stacking area will be in operation. Traffic signals will release HGVs on to the bridge at a rate of one every 30 seconds.”