The redevelopment, which is now set to cost £22 million, will 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.

With an original £16.8m budget, the project was delayed for “several months” last March 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.

Designed by Hoskins Architects, the updated plans will place the main entrance in Princes St Gardens, giving instant access to the Scottish collection, and much easier routes to the rest of the building as a whole. Additional display space will be created by transforming former offices, and the design will seamlessly connect the gallery with the gardens beyond.

A series of large windows, running the length of the new gardens-level façade, will fill the galleries with light and colour, and offer views the capital. Extensive new landscaping in the gardens will improve access, and the project will also create a larger shop, brand new café and more accessible restaurant.

Construction work on the project - which is supported by The National Lottery and Scottish Government - will begin in October. During the two years of construction, the gallery will be open as usual, though there will be some room closures as galleries are rehung, and the facilities in the gallery are refreshed.

The Playfair Steps at the rear will be closed for the duration of the building work to enable excavation works to the galleries directly beneath.

The first milestone will be the opening of a new coffee shop this Christmas, followed by a refreshed restaurant and shop, as well as new steps and accessible paths by Spring 2019.

Sir John Leighton, director-general of the National Galleries of Scotland, said: “This is a hugely exciting time for us, as we see our ambitious plans for the Scottish National Gallery getting underway. We will create the perfect showcase for the nation’s extraordinary collection of Scottish art, giving it room to breathe and showing it off with real pride to the world.

“This ambitious project will completely transform the experience of our visitors, creating a National Gallery that is even more open, engaging and inviting with new presentations of Scotland’s art in a setting that will be truly world-class. All will be welcome whether to come in and immerse themselves in the highly romantic paintings of the Scottish Highlands, to take part in a family day or just to linger, enjoy the views and maybe pick up one of our sketch pads.”