The £22 million revamp is set to be significantly delayed by the discovery of asbestos and “damp penetration” during work to convert areas originally built in the 1970s.

The first phase of the project to create new, world-class galleries to house the national collection of historic Scottish art was completed successfully in 2019.

This included a new entrance area level with East Princes Street Gardens, a refurbished café, restaurant and shop, and new landscaping and paths in East Princes Street Gardens to improve access to the gardens and the Galleries.

However, work on the next phase is now not expected to be completed until “the end of 2021” at the earliest, adding months to the previous forecast of “early 2021” and three-and-a-half years later than originally envisaged.

The redevelopment, set out with an original £16.8m budget to create a new suite of galleries that will be directly accessible from the adjoining Princes St Gardens, and provide a light-filled, new home for the gallery’s collection of Scottish art.

The project was delayed for “several months” in March 2017 after the National Galleries of Scotland announced that some aspects of the construction, namely the proposal to build out by an additional five metres into East Princes Street Gardens, were more complex and expensive than originally anticipated.

A revised scheme for the development unveiled two months later scrapped the extension into East Princes Street Gardens and pushed back the entire project a year for further design work.

Director-general of the National Galleries of Scotland, John Leighton, explained: “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 National Gallery. These include issues with damp penetration and inadequate drainage in and around the site.

“We are 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, when we will be showcasing a brand new space for our fantastic collection of Scottish art.

“In the meantime, the Scottish National Gallery remains fully open to the public who can also enjoy the new restaurant, café and much larger shop. Disruption to visitors and the public during this construction phase will be kept to a minimum.

“We look forward to unveiling our exciting new galleries which will be home to the world’s greatest collection of Scottish art, including some works that will be displayed for the very first time.”