The National Galleries of Scotland (NGS) said the delay of the planned extension to its headquarters into East Princes Street Gardens, which was due to begin this March, could last “several months”.

When the plans were approved in September last year, NGS said the Hoskins Architects-designed extension will radically improve access to its world-class collection of Scottish art.

However investigations into the project’s more complex designs revealed that the extension could be “potentially more expensive” than planned.

In a statement NGS said: “Work on construction of the new galleries was due to start in March.

“However there will now be a delay of several months to the full start on site. For the past six months we’ve been working with our main contractor Interserve on the detailed designs and various tender packages for the building work.

“It has become clear that some elements around the delivery of the construction work are more complex and potentially more expensive to implement than was originally anticipated.

“We therefore have to carry out some value engineering in the coming months in order to streamline some parts of the construction and bring the plans into line with our budget.

“In practice this means that we will be re-examining some of the specifications and construction methods for aspects of the design to ensure that the project stays within cost. Until this work on value engineering is complete we will not have a confirmed date for the start of construction but we are hopeful that we can begin work on site later this year.”

NGS added: “We will continue to work with stakeholders to ensure that there is minimal disruption to The Mound precinct during construction work. In the meantime, the Scottish National Gallery remains open as usual and the highlights of the Scottish collection are on display to the public.”

A spokesman for the Scottish Government, which is funding the development along with the Heritage Lottery Fund, said: “The project will increase and enhance public access to Scottish art and in the circumstances the National Galleries is rightly spending some further time in getting it right. The Scottish Government will continue to discuss funding support with the National Galleries.”