Campaigners against proposals to develop the Treasury Building at Inner Temple have set up an online petition and Facebook page to block the move.

After examining options to create new educational facilities, the Inner Temple’s Executive Committee has proposed a scheme that will add an additional floor to the 1950s building and use two of the western rooms of the library to create a tiered auditorium. A mezzanine floor at the gallery level of the library will create a second additional storey.

A statement from the Inn said: “There is an urgent need to have educational facilities that are fit for purpose as the current bench apartments are not suitable for training purposes.”

Those opposed to the plans say they will “devastate” one of the finest law libraries in the world, that has existed since 1440, reducing the floor space by 58%, cutting the reader spaces by 50% to 49, displacing 25,000 books and closing the entire library for a year and a half while the work is done.

They prefer an option that involves adding a fourth floor to the existing Treasury Building to provide training and meeting rooms and a 99-seat auditorium.

In a letter to the Executive Committee, four masters of the Inner Temple Library expressed concern that the “radical” proposal was driven by “planned commercial enterprise” ancillary to the Inn’s chartered purpose, and had “nothing to do” with the library’s layout or function.

A final decision will be made at a specially convened meeting on 21 October.