A National Galleries of Scotland boss has apologised after admitting part of a multi-million pound revamp of its Edinburgh attraction is running several months behind schedule.

A section of the £22m National Gallery makeover was expected to be complete by the spring, however part of East Princes Street Gardens and the gallery's cafe and restaurant will now be out of bounds until the start of the Edinburgh Festival in August.

The new-look garden path and landscaping, which has been designed to help those with mobility impairments, wheelchairs and prams, is said to have been "a complex engineering feat" which had "taken slightly longer than programmed".

The overall project - which will showcase the gallery's rich collection of historic Scottish art including masterpieces by Sir Henry Raeburn, The Glasgow Boys, Charles Rennie Mackintosh and the Scottish Colourists - is still on target for completion in early 2021.

Sir John Leighton, director-general of the National Galleries of Scotland, said: "I want to extend a big thank you to the residents and visitors of Edinburgh during the construction works in East Princes Street Gardens.

"We apologise for any inconvenience and we appreciate you bearing with us whilst we make these transformative changes."

A Scottish National Galleries spokeswoman added: "It's a complex piece of engineering, and our contractors have had to adapt to some of the challenges that are inevitably uncovered in major construction projects like this.

"Not only is the Scottish National Gallery a category A building, set above a major railway line and in a UNESCO heritage site, but we also discovered some utility pipes which had not been documented and were required to be rerouted."

The work has been scheduled to accommodate crowds and attractions during notable events.

The Edinburgh Marathon in May will still run through East Princes Street Gardens and the Mound precinct as a route will be cleared for use.

All the trees that were controversially felled last autumn will also be replaced. These will be divided between the east and west gardens, as well as Lauriston Castle and Saughton Winter Garden.

Sir Leighton added: "As previously announced, the overall project is on schedule to be complete in early 2021.

"By then we will be ready to reveal an amazing new suite of galleries, bursting with light and colour with new views out into Princes Street Gardens.

"Previously only one in six visitors found their way to the dark and tucked away Scottish Galleries, so this will transform the presentation of the world's greatest collection of Scottish art."