Cllr Robb, who put forward the idea to council, said the referendum was "a final opportunity to resolve significant differences of opinion".
The new council suspended the controversial designs for the Square at its first meeting at the end of May.
Designs have been criticised for not having enough green space.
Council leader Roddy McCuish said: "The council decided there should be one last opportunity for the public to be consulted and offered a genuine choice on the design proposal. The funding is ring-fenced and ready to be drawn down to progress whatever option is selected and taken forward by the council."
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The informal referendum - which avoids the costs associated with a statutory referendum - will take place on Monday June 25 between 8am and 8pm in the main hall at Helensburgh's Victoria Halls.
Details of the three options included in the referendum will be available from Monday 18 June, on the council website and from Scotcourt House and Helensburgh Library.
In order to be eligible to vote in the referendum you must be a resident in Ward 9 Lomond North, Ward 10 Helensburgh Central and Ward 11 Helensburgh and Lomond South. You must be aged 18 or above on or before 25 June.
The results of the informal referendum will be considered by the council when looking at how to progress the improvement works.
The next stage is the contract procurement for the redevelopment.
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