SCOTTISH Opera's £14 million revamp of its Theatre Royal home will not be completed until late September, a delay of five months.

Productions of Madama ­Butterfly and a special Glasgow Commonwealth Games show, Anamchara - Songs of Friendship, written by Alexander McCall Smith, will go ahead as planned but the construction of the building's landmark new foyer has hit unexpected snags, the opera's chief executive has revealed.

The building of the dramatic extension, designed by architects Page/Park, has been slowed by bad weather over the winter, ­problems caused by its tight site on the corner of two busy Glasgow streets, and unexpected issues with the fabric of the Theatre Royal building.

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The new foyer, a cylindrical addition that will include a roof terrace, bars and cafes as well as new entrances to the main theatre auditorium, was meant to open in May, a deadline that was moved back to July - but now Scottish Opera will not unveil the building until the autumn.

Audiences will still be able to attend events in the theatre but portable toilets will remain in use outside.

General director Alex Reedijk said the extended timetable on the construction of the extension would ensure it was of the ­"highest quality".

There will be extra cost caused by the delays, he said, but this has not yet calculated - Mr Reedijk said he expected it to be in the low thousands of pounds.

The contractor for the revamp, Sir Robert McAlpine, informed Scottish Opera last week that it was very unlikely to be completed by late July and the opening of the Commonwealth Games.

"The good news is that the ­auditorium is done, and the old foyer is done, so all the shows can be preserved, the performances will be unaffected," he said.

"But it was a case of sitting down with the contractor and saying 'I think all of us need to take a reality pill - we can't have a situation is where the quality slips away, because we are building this for the next hundred years' - so let's do it properly. We had a really good run at [a July deadline] but we haven't been able to quite get there, so let's be honest about it."

He added: "I am disappointed, but what I am not disappointed in is that the bits that are finished are amazing."

The new foyer, which has a dramatic spiral staircase at its centre and commanding views of north and west Glasgow from its roof terrace, is being built on to the north end of the Theatre Royal.

Mr Reedijk said: "Essentially it is the joining an old building to a new building, and any time we looked at that back wall, the north gable, there was another thing that we had not expected.

"It is a 150-year-old wall that's had so many interventions over the years, it's had so many uses, interventions, apertures opened and closed, and particularly where we have been physically joining old and new together we have bumped into so much."

Other problems have included finding unexpected power cables under the pavement outside the Theatre Royal, which had to be dug up and relaid.

Mr Reedijk said he thought it was best not to open "bits of the building" at a time. "We are intending to be done at the end of September," he said.

"We think there might be some modest cost implications, we think in the thousands, but it is too early to comment."

The tightness of the 480 square metre building site has presented its own problems. "The site is bound on two sides by two busy roads, you are backing onto a jolly old building and a brand new building," he said.

"You cannot block off the road [Hope Street] and we can only work on Cowcaddens Road on certain hours of the day.

"So that has slowed it down a little bit and when you hit difficulties that slows it down again."