Westminster planners have approved the controversial plans by New York studio Selldorf Architects to remodel the postmodern Robert Venturi and Denise Scott Brown-designed Sainsbury Wing at the National Gallery.

Last night the planning committee in Westminster, London, approved the Selldorf Architects' revised plans to alter the National Gallery's 1991 extension. The plans have been heavily criticised as an "act of vandalism", including by eight former RIBA presidents.

"Scheme will cause substantial harm"

Following the decision, conservation group Twentieth Century Society (C20) stated that the proposal will "cause substantial harm" to the building, which was Grade I-listed in 2018, and suggested that it would contest the decision.

"Westminster planners have just approved the destructive plans for the National Gallery Sainsbury Wing," wrote the Twentieth Century Society on Twitter.

"The scheme will cause substantial harm to a Grade I-listed building and C20 strongly objected. Has the 'Second Battle of Trafalgar' just begun?"

The plan to revamp the building will see the entrance remodelled and the dark glass replaced with transparent panes on the facade.

An espresso bar will be added on the ground floor and a cafe, seminar room, archive and staff offices added on the upper floors.

"We have retained the essential quality and features of the Sainsbury Wing"

Proposed as part of the gallery's NG200 project to mark its bicentenary in 2024, the plans have been subject to two public consultations.

"The exchange we have had over the last 18 months with all those who care about the gallery, not just in Westminster, but around the world, is echoed in our plans and its dialogue with the existing buildings," said Selldorf Architects principal Annabelle Selldorf following the decision.

"We have retained the essential quality and features of the Sainsbury Wing whilst creating a space and arrival sequence that is welcoming to all."

However, ahead of the decision, original architect Scott Brown made a plea for the alterations not to be made as they would damage the "carefully orchestrated" entrance route.

"Much of this carefully orchestrated movement of the visitor through the building will be lost should the current proposals be approved and the destructive, irreversible demolitions be allowed to proceed to this Grade I-listed building," she wrote.

"Thanks to the guardians of our heritage"

Venturi and Scott Brown's Sainsbury Wing is one of the country's youngest Grade I-listed buildings – these are deemed to be "of exceptional interest" with only 2.5 per cent of all listed buildings given this level of protection.

Critic Hugh Pearman spoke out against the decision by heritage authority Historic England, which oversees the listing system, to withdraw its objection to the proposals following the revisions in October.

"Looking hard at Historic England, who on November 6 decided that the revised Selldorf scheme was just fine so far as they were concerned and withdrew their objection," Pearman said on Twitter.

"The case was effectively lost there and then, thanks to the guardians of our heritage."

Scott Brown echoed this sentiment in her plea to refuse the planning, suggesting that this decision could impact how other Grade I-listed buildings are treated.

"They [the architects working on the project] should be exceptionally pleased to have the opportunity of working with a building of the highest level of listing and of not traducing that standard by what they do as it will affect all future cathedrals," she said.

"What this architect does now in this Grade I-listed building could endanger all the others."

Following the planning approval, the National Gallery is aiming to complete the renovation by May 2025.