Plans for a Forth Bridge bridge walk were given the green light by councillors in March.
The plan to open the bridge to an estimated 85,000 visitors a year will be delivered in two phases.
This will involve the construction of a bridge walk and reception hub on the south side of the Unesco World Heritage site at South Queensferry costing up to £15m.
The second part of the plan costing £20m will involve building a visitor centre beneath the bridge’s northern Fife tower.
This will house two glass lifts, which will slowly scale the structure to the viewing platform at the top.
The designs envisage the visitor centre with a glass roof to provide a dramatic view of the structure from below.
Firms have until 3 August to complete prequal documents. Four firms will be shortlisted to tender for the work which is due to get underway before the end of the year.
Click here for procurement documents.