Under a blueprint aimed at safeguarding the venue’s future for another 50 years, radical improvements will be carried out to its stage, auditorium, backstage areas, bars and cafe areas, The Scotsman has reported.

The City of Edinburgh Council and the Heritage Lottery Fund (HLF) would contribute £5m each under a rescue plan which see a similar amount generated via contributions from theatregoers.

The charitable trust, which runs the theatre on behalf of the council, needs a further £10m from a public appeal to ensure work can begin as planned in 2021.

Councillors will be asked next week to approve the £5m backing in principle to allow a bid to be lodged with the HLF later this year.

The theatre, which dates back to 1906, would close for 18 months to allow the overhaul to be carried out, though it is not yet known whether the timetable of work would affect two summer festival periods or two pantomimes.

The first major overhaul of the “Grand Old Lady of Leven Street” since the 1950s, when its upper balcony was removed, is expected to operate as a daily visitor attraction, with guided tours and an exhibition of archive material.

The theatre’s currently 1,350 capacity is expected to be reduced by around 200 to allow the back of the upper stalls to be moved forward and new corporate hospitality spaces to be created, including a possible glass roofed area boasting views across the city.

A planned £14m revamp in time for its centenary was shelved after it was turned down for backing by the HLF and the then Scottish Arts Council. A £2.6m scheme for urgent repairs, ensure the building was wind and watertight, and install new seating in the stalls and dress circle was carried out in 2012.

However, a new report for the city council, which has owned the building since 1969, warns that it is “in need of major development to bring it up to 21st century standards”.

It states: “Backstage redevelopment is essential to meet current legislative requirements and to attract modern touring companies. The exterior and interior of the building are in a poor state, and facilities are in need of modernisation. Access is extremely difficult for visitors with limited mobility.

“The ambition is to modernise and preserve the King’s for at least the next 50 years. The redevelopment would give a new lease of life to a beautiful and important historic building, and an asset to the city and local community.”

The council report warns the HLF is “highly unlikely” to support the project unless the council matches the £5m contribution from the trust, which is being generated from a £1.50 levy on tickets.

Duncan Hendry, chief executive of the Festival City Theatres Trust, said: “Access to the theatre is very poor at the moment and the backstage facilities are not in great shape. There’s a risk of gradual decline, with attendances dropping and companies not coming here anymore because the facilities don’t match their expectations.

“On a more positive note, Edwardian and Victorian theatres that been done up around the UK tend to be reinvigorated by that process.”