A £22m revamp of the Scottish National Gallery is set to run over budget and be delayed by three-and-half years after asbestos was discovered.

The multi-million pound overhaul of the 19th century gallery in Edinburgh's Princes Street Gardens, which houses some of the country's most important pieces of art, involves the creation of new galleries in an extension built in the 1970s.

The ‘unexpected defects’ relate to the final phase of work, to convert former offices and storage facilities into gallery spaces.

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However, according to The Scotsman, deadly asbestos, damp penetration and inadequate drainage were found in one of the new exhibition spaces while work was underway.

Extra work will have to be put in to ensure that the building, where work by artists like Allan Ramsay, Joan Eardley, Phoebe Anna Traquair, Sir David Wilkie and Samuel Peploe will be displayed, is “fully protected and safeguarded”.

The final work on the National Gallery, which first opened in 1859, is now not expected to be completed until “the end of 2021” at the earliest.

The project marks the biggest change at the attraction since an underground link was created in 2004 to join up the National Gallery and Royal Scottish Academy buildings.

The overhaul of the gallery, which is being partly funded by the Scottish Government and the Heritage Lottery Fund, has been dogged by problems since the project was first announced in 2015 with a £15.3m price tag.

Its design had to be significantly scaled back due to the soaring cost of initial plans to extend the gallery building into East Princes Street Gardens.

By the time work got under way in October 2018 its completion date had been put back to “early 2021.”

However the project was hit by further problems last year.

It emerged that work on a new main entrance to the gallery and a major re-landscaping project in the gardens - including a new zig-zagging pathway - was running several months behind schedule.

Also, a new cafe, restaurant and shop all opened later than planned last year.

Sir John Leighton, director-general of the National Galleries, said: “In the course of this work it has become apparent that there are some unexpected defects within the 1970s building which sits beneath the original nineteenth-century gallery.

“We’re now addressing these issues to ensure that the building is fully protected and safeguarded for the future, so that it can better withstand any future extremes of weather.

“In order to accommodate this extra work we have therefore revised our timescales for the project.

“We are now aiming to complete works on the new gardens level galleries by the end of 2021.”

A spokeswoman for the National Galleries said: “Several issues were discovered when the 1970s building was fully stripped back to its core concrete structure.

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“These include damp penetration, inadequate drainage in and around the site, asbestos, and unexpected obstructions in the surrounding site.

“They must be addressed to ensure the building meets the highest standards in order to safely accommodate Scotland’s national collection of art and to protect it for the future.

“The extra work that we now have to carry out will add some further cost in time and this is currently being reviewed.”