Edinburgh faces a bill of an extra £860,000 to ensure its premiere music venue, the Usher Hall, is in a useable state for this year's International Festival.

The troubled multi-million pound refurbishment of the auditorium is set to cost City of Edinburgh Council even more money in the future, with "considerable" costs due to delays in the completion of the project, according to a report to be considered by city councillors next week.

Last week it was announced that the winter seasons of the Royal Scottish National Orchestra (RSNO) and the Scottish Chamber Orchestra (SCO) would not be staged in the venue because of delays to the second phase of its refurbishment.

Up to £400,000 is being paid to the orchestras in compensation for loss of the venue, and a progress report written by Jim Inch, director of corporate services, for the council's culture and leisure committee confirms that it has cost more than double that to get the venue ready for this year's festival.

It says the £864,000 was spent on getting the Usher Hall ready for the festival "as a matter of urgency in order to secure the delivery of the EIF 2008".

"Without this action, the delivery of the EIF 2008 programme would have been compromised," it reads.

"It is anticipated that further costs will be incurred due to time extensions to the contract necessary to accommodate the festival."

The report also says that the extensive delays to the Usher Hall refurbishment, due to a range of difficulties encountered at the site, would lead to more costs and possibly endanger future building projects in the city - although these are not specified.

The new charges come on top of the £20m bill for the second phase of the Usher Hall's development.

The refurbishment has been beset with setbacks and problems since late 2007, when a series of structural issues, among them "unexpected rock formations", were discovered.

The original plans for the revamp of the hall, whose acoustics are considered some of the best for live music in Europe, have already been altered once.

The report says much work remains to be done at the concert hall, which will host 23 concerts at this year's festival.

Deidre Brock, the council's leader of culture, said: "The Usher Hall is critical to the programming and box office success of the EIF. This investment will allow us to transform the Usher Hall from a building site into a concert venue in a matter of weeks so the EIF can present another fantastic summer festival season.

"Once the festival is over our contractors will be back on site working flat out on the project."

Jonathan Mills, EIF director, said: "We are very grateful to the city for working so hard to ensure the festival can go ahead despite these difficulties and delays."

The RSNO will instead perform in the Edinburgh Festival Theatre for the 2008-09 season and the SCO will move concerts to the Queen's Hall, Festival Theatre, and Greyfriars Kirk.

The culture and leisure committee of City of Edinburgh Council will discuss the new report into the state of the Usher Hall on June 24.