Managers at Dan yr Ogof oppose the scheme

Managers of the showpiece Dan yr Ogof Caves in the Swansea Valley said the new scheme will be the "nail in the coffin" for many private operators.

They said some attractions - already reeling from the blow of foot-and-mouth - would not be able to compete.

As well as the main national museum in Cardiff, the free visitor sites include the Big Pit mining museum at Blaenavon in south east Wales and the Welsh Slate Museum at Llanberis in north Wales.

Also open to the pubic for free from 1 April are the flagship Museum of Welsh Life at St Fagans, near Cardiff, the Roman museum at Caerleon and the Turner House Galley in Penarth.

The national museum is marking the beginning of the free entry era by the symbolic release of balloons at its main site in Cathays Park, Cardiff.

A Cardiff family who have never visited the museum together will take part in the launch.

NMGW director Anna Southall said it was a great day for Wales.

"Thanks to the National Assembly's support, our national museums will be free for several months before full free admission will be introduced in England."