A previously obscure building designed by the young Charles Rennie Mackintosh, which could have been a prototype for his most famous creation, is to be re-introduced to the public as part of a festival celebrating his legacy.

The Mackintosh Club in Helensburgh was designed in 1893/1894 by Charles Rennie Mackintosh, then only 25, for Honeyman and Keppie.

Its upper floor, previously a billiard and committee room for the Helensburgh and Gareloch Conservative Association, had lain empty since the early 1970s.

However in June this year this upper floor was purchased for a five figure sum by two Scottish architects, Bruce and Nicola Jamieson, who run architects firm Pure Green Space.

Now the Jamiesons have opened the upper floor to the public for talks, and intend to create a museum and gallery dedicated to the 'Glasgow Four' - Mackintosh and his peers Herbert MacNair and artists Margaret and Frances MacDonald.

The group's relationship was more than friendly or professional - Mackintosh married Margaret MacDonald, and MacNair married Frances - and the four represented a seminal creative force in Scotland in the early years of the 20th century.

Bruce Jamieson said there is a two year plan for the building, which will make it "relevant again" and build towards the 150th anniversary of Mackintosh's birth in 2018.

He added: "There is so much more that can be done with the building.

"Mackintosh was so young when he designed it, but there are elements of the symbolism, that was like rocket fuel for the Four at this time, that can be seen in its elevation."

The Mackintosh Club is to be part of the forthcoming Mackintosh Festival, which runs from October 1 to 31.

Nicola Jamieson said the couple had been trying to buy the "unknown jewel" for several years, and already several Mackintosh experts have visited the site.

They have secured documents which prove, she said, that the young architect had his hand in the designs in the building, which building stands in one of the main commercial streets of Helensburgh, and has club rooms above ground-floor shops.

Ms Jamieson believes the light-filled upper room was possibly a prototype for Mackintosh's designs for the famous studios in Glasgow School of Art, which he designed after the Conservative Club.

She added: "It is definitely a massive undertaking but it is extremely exciting, we are so delighted we have it, and a place to celebrate the Glasgow Four has never been done before."

Now in its fifth year The Mackintosh Festival is a month-long programme of arts and cultural events to celebrate the life of Scotland's most famous architect, designer and artist.

Last year the festival's events attracted 100,000 visitors.

Other events at the festival will take place at the Hill House, also in Helensburgh, the House for an Art Lover in Glasgow, The Lighthouse, the Willow Tea Rooms, Scotland Street School, Kelvingrove Art Gallery and the Queen's Cross Church.

The Glasgow School of Art will host 'Disappearing Glasgow'by photographer and filmmaker Chris Leslie and there will be talks on the ‘State of the Mack’ by the Mackintosh Building Restoration Team.

The Mackintosh Festival is organised by Glasgow Mackintosh, a heritage group established in 1980 by curators and managers of Mackintosh buildings and collections in and around Glasgow.

There will also be an event at the St Enoch Centre in Glasgow, Mackintosh The Innovator.

It will be an "immersive 3D experience that allows visitors to design and interact with Mackintosh space".

The event will take place every weekend during the festival and will be free.

The 3D show has been created with Wireframe Immersive.