The Georgian Group has lodged objections to the creation of an arched opening at the Royal Institution, saying this would ruin the building's last remaining original architectural feature.
The architect's refurbishment of 18-21 Albemarle Street, off London's Piccadilly, is being considered by planners at Westminster council tomorrow evening.
But the Georgian Group's southern caseworker, Josephine Brown, believes the grand stair entrance - one of the institution's 'finest spaces' and attributed to John Carr, would be 'significantly compromised' by proposals to link the front portion of the institution to its rear.
She said: 'The space, as designed, would have originally had a flat wall at that point and that is important for articulating the stair. It's one of the last remaining spaces that hasn't been messed about with. There's no real purpose to knocking through given that there are doors beneath the stairs.'
English Heritage initially registered its own disapproval to the proposal, but has since been talked round by discussions with the architect. The council's planning officers have recommended approving the scheme ahead of tomorrow's meeting.
The Royal Institution has been continuously housed in the same buildings since 1800 and contains the oldest research laboratories in the world.
Within its walls 10 of the hundred or so known chemical elements in the universe were discovered. Fourteen Nobel Prizes are associated with Royal Institution scientists.
To keep the institution operational during construction works, the project has been phased with a target start date on site of November 2005 and completion by Christmas 2008.