In August, thieves broke into the Museum of Costume and Textiles via a back door and stole £33,000 worth of pottery and textiles. The museum is only open for guided tours and research purposes.

The Science Museum in London also became a target in July when thieves broke into a display case during opening hours. They took 14 microscopes worth more than £150,000. A spokesman for the museum said the items included an important 17th-century French hand microscope and an ivory microscope from the 18th century.

The front glass of the case, which did not feature an alarm, was levered out when the patrolling warden was absent for a short time. There was no CCTV in the gallery. Both crimes are now being investigated by the police.

Tim Boon, the head of collections at the Science Museum, told Museums Journal: 'It is disturbing that a small number of determined criminals have selfishly taken advantage of our public role in this way.'

The incidents follow a theft at the Science Museum and three at the Victoria and Albert Museum in London last year. It is not known if the crimes were opportunistic or planned.

Iain Slessor, the national security advisor at the Museums, Libraries and Archives Council (MLA), said of the latest Science Museum theft: 'Taking 14 microscopes is quite a big event. I would suggest that there was some planning involved.'

Slessor keeps a record of museum and gallery thefts each year, but only if the museum reports them to him. In the current year (2005/6), so far three incidents have been reported, including the latest one at the Science Museum.

The other two took place at county museums in England. One was during a burglary where the alarms were activated, said Slessor. The other, where a £15,000 bronze statue was stolen, took place after a man was seen acting suspiciously, but the attendant was distracted by a lift panic button.

Last year (2004/5), there were 12 reported thefts - the same number reported in 2003/4. The figures show a steady decline from 1995/96, when there were 54 reported thefts, to 2000/01 when 22 thefts were reported to the MLA.

Sandy Nairne, the director of the National Portrait Gallery who will be delivering a paper at next month's AXA Art Conference on the investigation into art theft, said it is a matter of finding the balance between deterrents and protection.

'It is a question of how far we should use deterrents such as CCTV or adopt mechanisms that shop assistants employ such as welcoming someone when they enter a shop to make it clear that they have seen you.'

Nairne added: 'People [in the sector] find it difficult to talk about complex and conflicting security ideas, but it is important that the issues are discussed.'

Felicity Heywood