There are to be further delays to the redevelopment of one of Scotland's leading concert venues, the Usher Hall in Edinburgh.

The A-listed concert auditorium renowned throughout the music world for its fine acoustics and architecture, is undergoing a £30m revamp, and is closed until the Edinburgh International Festival in August.

The City of Edinburgh Council has confirmed that the redevelopment is going to take longer than previously thought, and no exact date has been set for its completion, beyond a general target of spring next year.

The Usher Hall is a key concert venue in particular for two national companies, the Royal Scottish National Orchestra (RSNO) and the Scottish Chamber Orchestra (SCO), and any threat to their use of the hall during the winter season would be a serious blow to both.

However, the council said that "at this stage" there is no concern over the use of the hall by the SCO and the RSNO for their next winter seasons.

The RSNO has missed a season in the Usher Hall and has had to stage concerts in the city's Festival Theatre.

The delays have been caused by a need to reinforce the foundations of the building and provide additional support to some internal walls.

The council is no longer taking bookings for the hall after the August festival season is over.

A spokesman said: "This work means that the full project will take longer to fully complete but there is no concern at this stage over the SCO and the RSNO's use of the venue for this coming winter season. However, we will not be taking additional bookings due to the risk of further delays and costs.

"It may be the case that some temporary public facilities, such as toilets and box office, are put on site for the season, but we expect the hall itself will be operational. A planning team is being set up to ensure that the needs of the orchestras and audiences are satisfied."

Last night Simon Woods, the chief executive of the RSNO said: "We are thoroughly supportive of the renovation of the Usher Hall which will produce a superb facility for the RSNO and its audiences.

"It is understandable given the age of the building that unforeseen challenges arise and we are confident the City of Edinburgh Council is doing everything in its power to minimise disruption next season."

Work resumed on the Usher Hall as soon as the last Edinburgh International Festival finished, and the council said there were no concerns for its reopening for this year's event.

A spokeswoman for the festival said: "We do not have any concerns about the Usher Hall.

"We have been assured that the council is doing everything possible to make sure it goes ahead and we are sure that it will."

In November, the council updated the plans for the redevelopment of the hall after the discovery of a "range of difficulties" with the construction site.

A dig into the basement revealed that the hall's foundations would need to be reinforced to support a new glass wing, the most visible change to the hall.

In addition, the discovery of an "unexpected rock formation" led to structural engineers ordering deeper pilings to ensure the future integrity of the building.

As part of the revamp, the glass wing will be added to the south side of the venue along, with a new stairwell and public lift to all floors, an education space and upgraded public and performer facilities.

Originally it was planned that there would also be structural changes to the north side of the building, including a new lift and a stairwell, but these will no longer be included - the council said that £2m would have been added to the costs of the project if they were built.

The project to upgrade the hall has come in two phases - Phase I of the Usher Hall refurbishment project was completed in December 2000, at a cost of £9.75m. Phase 2, the current work of, has been costed at £20m.

The first phase provided the Usher Hall with new mechanical and electrical installations, upgrades to the fabric of the auditorium, a new stage and removable seating to allow promenade concerts.

The current phase will provide education space, the glass wing - housing a cafe, new box office, and offices - toilet and cloakrooms and better backstage facilities.