Recent coverage of objects missing from museum collections is "misleading and damaging", the director of the Museums Association (MA) has said.
Collections management has been under scrutiny following the announcement in August of alleged thefts from the British Museum.
Since then, Freedom of Information requests by the media have revealed that thousands of objects are unaccounted for in a number of UK museums.
A report by BBC News this week showed that 1,921 objects are missing from the collections of Amgueddfa Cymru - Museum Wales. These include Roman tiles and coins, a bronze age sword, axe and javelin head, royal medals, and crockery.
Amgueddfa Cymru told the BBC that a significant number of the items were duplicates of which there were better examples within the collection, and that it expected most of the items to be recovered as it worked through the collection.
Other news outlets have reported that 560 items have been mislaid, destroyed or stolen from the collections of Imperial War Museums (IWM), including drawings from the Illustrated London News, miniature replica weapons and wartime posters. In total IWM cares for more than 155,000 three dimensional objects, 11 million photographs and 20,000 papers.
Meanwhile 540 items are unaccounted for in the collections of the Natural History Museum, including parts of a dinosaur fossil and microscopic slides. The museum holds the largest natural history collection in the world, with around 80 million objects.
Museum leaders have warned that the coverage is misleading, saying that missing items in collections numbering millions of objects is not unexpected, and that the movement of objects is a normal part of museum work.
MA director Sharon Heal said: "These stories are misleading and damaging to the reputation of museums. It is unsurprising that, with collections amounting to tens of millions of items that have been collected over many decades, some items might be missing or unaccounted for.
"As we said in our statement following the disclosure of theft from the British Museum, theft from collections by people who work in museums is, fortunately, incredibly rare."
Heal added that collections documentation has been hampered by years of systemic cuts to museum budgets.
"Museums have been working hard to document and digitise their collections over many years but unfortunately the systemic underfunding of the sector over the past 10 years in particular has undermined their ability to do this," she said.
"Funding cuts, restructures and redundancies have led to a loss of expertise and a weakening of the normal systems of checks and balances, as well as slowing down digitisation and documentation programmes, and collections research."
Heal added: "Movement of objects into and out of museums, whether through loans, outreach, acquisitions or disposals, is a normal part of what museums do.
"The Museums Association recently published Off the Shelf, a toolkit to support the ethical transfer, reuse and disposal of collections, which will support museum workers to think more proactively and dynamically about collections and to make decisions with communities about the best use of our rich and diverse collections.
"Looking to the future we would like to see investment in museums – in the people who work in them and in collections and buildings – so that we can continue to safeguard and share collections with communities across the UK."