The event will also launch a series of Heritage Open Days in which the city's most historic buildings throw open their doors to the public.

The Leeds Library is the oldest surviving proprietary subscription library in the British Isles, having been established and funded by its members in 1768.

It is based in a Georgian building in Commercial Street in the retail heart of the city centre.

It was founded by 104 local people. Each subscriber contributed an initial guinea (£1.05p) and paid an annual subscription of five shillings (25p) to fund the library and expand its collection of books.

The most famous and influential of the original subscribers was Dr Joseph Priestley, the minister of Mill Hill Chapel and discoverer of oxygen.

The current library was built in 1807.

The library has been awarded a blue plaque by Leeds Civic Trust, guardian of Leeds' archeological heritage.

The plaque was being unveiled today by the Deputy Lord Mayor of Leeds, Coun Alan Taylor.

Following the unveiling, Leeds Heritage Open Days will be launched by Coun Ann Castle.

The library is one of dozens of historic buildings which will be opening their doors to the public.

George Whitehead, President of the Leeds Library, said: "The Leeds Library is one of Leeds' oldest and finest historic institutions.

"It is still going strong after 240 years and I am delighted that it will be receiving visitors during Leeds Heritage Open Days.

"The blue plaque will draw attention to our architecturally outstanding Georgian building in Commercial Street and remind people that, though the library is historic, it is also a thriving modern library which aims to play a full part in the life of Leeds."

The four-day Leeds Heritage Open Days will see the doors thrown open at historic houses, green spaces, theatres, libraries, museums, railways and places of worship in the city.

Experts and enthusiasts will also lead walks and tours highlighting aspects of the heritage of Leeds and its district.

Leeds Heritage Open Days are part of a nationwide annual event, but the Leeds event is the country's largest Heritage Open Day outside London.

This year families will be able to "get behind closed doors" at the Royal Armouries, Leeds Museum's new Discovery Centre, the City Varieties, the Carriageworks Theatre and the West Yorkshire Playhouse, and many more.

Ancient history will be on offer at Barwick's Iron Age Fort and in Gipton Woods.

* Full details of Heritage Open Days events can be found on the Heritage Open Days website, www.heritageopendays.org.uk