Ministers will announce their decision on a replacement Forth crossing on Wednesday, BBC Scotland has learned.
The current Forth crossing is deteriorating
It is thought that the cost of building a tunnel under the Forth - estimated at more than £2bn - may be prohibitive.
However, the £1.5bn price of the other shortlisted option, to build a new bridge, will have major implications for future spending plans.
The SNP cabinet is considering both at its weekly meeting, ahead of the outcome being revealed to MSPs.
The government has been under pressure to take a decision because of the deteriorating state of the old bridge.
There is a risk that corrosion in the suspension cables may mean a lorry ban by 2013.
The timetable for debates at Holyrood is being amended to make room for the decision, which will be made in the form of a parliamentary statement.