The stone was carved from the Welsh mountains

Some experts said the inner ring of the monument is made of bluestones, which were carved out of the Welsh mountains before being heaved on sleds and boats to the Wiltshire site.

While the sinking was just the latest in a series of disasters which has plagued the project, organisers are to continue with the 240-mile long journey despite it being criticised as a waste of money.

Rhys Sinnett, Plaid Cymru's prospective parliamentary candidate for the area, said the stone should be left where it was.

He asked whether the effort was worth �100,000 of Lottery money and he has been supported by local Conservative leader Hugh Luke.

Phillip Bowen: Mishap not the end

Volunteers had been attempting to transport the stone on two supporting curraghs - traditional longboats - towards Tenby.

When the crews returned they found the bluestone had slipped its ties and sunk.

It is the latest mishap to hit the project which has attracted a �100,000 grant from the Lottery heritage fund.

But Philip Bowen of the organisers Menter Preseli insisted it would not signal the end of the trek.

Target

The problems the project have encountered include:

• Volunteers managed just one mile on their first day - a third of the hoped for target.

• A lack of volunteers caused another day of stone pulling to be called off.

• The stone's final pull overland from the Preseli Mountains to the River Cleddau was held up while a search was mounted for the sled, after it disappeared.

• The operation had to be postponed after the stone failed to slide into position, mid-stream in the River Cleddau.