The Liberal Democrats will pledge in their general election manifesto to guarantee continuing free access to selected national museums and galleries while offering greater autonomy to the institutions.

Danny Alexander, the Lib Dem chief secretary to the treasury, will say that guaranteeing free access will ensure that people from all backgrounds can continue to enjoy the "great institutions".

The Lib Dems will say that they will continue to offer government funding to 15 national museums and galleries on the basis that they continue to grant free entry, as they have since 2001. The Lib Dems would also expand a four-year pilot introduced by the coalition to offer greater financial flexibility to the museums.

The 15 national museums include the British Museum, the National Gallery, Tate Modern and Tate Britain.

Alexander said: "Our museums and galleries are some of the best in the world. They are a source of inspiration and education for millions across the country. As Liberal Democrats in government we've played our full part in making sure they have the funding and operating freedoms to widen access to all parts of society. The fact that attendance is at record levels and that last year was the first time that a majority of people had visited a museum or gallery shows that free access is a policy that works.

"We're now committing to maintaining that free access in the next parliament so that people from all backgrounds and income groups can make use of these great institutions without concerns about the cost. I see this as another way of helping make sure that as many people as possible have the best chance of getting on in life."