SEVERE damage was caused to the Erskine Bridge over the Clyde yesterday after a #70m oil platform scraped along its underside while being towed down river.

The 50 metre-high rig's passage under the bridge was not blocked and most lanes on the bridge were allowed to stay open to traffic while engineers carried out a structural inspection.

However, it was decided to close the bridge, which connects the M8 with the A82 Glasgow to Balloch road, to allow a ``full engineering inspection'', which is expected to take up to three days.

The bridge, built at a cost of #10.5m and opened in July 1971 by Princess Anne, will be inspected today by a team of specialist bridge assessors from London and Gloucestershire.

It is expected that the manufacturers, tug companies, Clyde Port Authority and the bridge authorities will launch an investigation into the incident.

It is understood that the height of the rig will feature prominently in the investigation.

Other factors to be examined will include the route of the vessels towing the rig, the movement of the tides and wind speed at the time of the accident.

It is understood that the port authority is responsible for piloting loads and checking the tides. It is believed that a team of port authority hydrographers monitored the tides yesterday morning before the rig was taken down the river. There is some dispute over whether the manufacturers or the tug companies are responsible for the load during transit.

It is claimed that the stretch of river below the bridge is regularly dredged and there have been no problems with silt deposits.

It is uncertain how long the bridge will remain closed. Police said alternative routes, such as through the Clyde Tunnel, would be signposted for motorists until the bridge reopened.

Any delay will lead to even worse traffic headaches for motorists already braced for motorway delays until the middle of next month on the M8 in Glasgow caused by extensive roadworks which started without major incident this weekend. The bridge normally carries about 20,000 vehicles a day.

The 6500-tonne rig was being towed to Texaco's Captain field, 90 miles north east of Aberdeen, from the UiE construction yard at Clydebank when it passed under the

bridge about 7.45am yesterday.

Witnesses on the bridge told of hearing a ``noisy screeching'' as it scraped the underside.

One resident, who lives under the bridge in Old Kilpatrick, said he was woken by the sound of ``crushing metal''.

He said a number of rigs had passed under the bridge without incident in the past.

Mr Ian Hamilton, head of roads with Renfrewshire Council, which manages the bridge on behalf of the Scottish Office, said the construction had left a 10.6-metre score on the underside of the bridge, causing ``severe damage.......enough for us to close the bridge. We don't do that lightly.''

He denied that there had been any delay in closing the bridge to allow traffic heading for the Oasis concert at Balloch to clear. The volume of bridge traffic - which had been expected to double throughout the day - had not materialised.

``There was no question of safety being compromised. We inspected the bridge immediately. As soon as we established there was damage we proceeded to close it,'' Mr Hamilton said.

He said the immediate future of the bridge operation would remain uncertain until bridge assessors had completed their inspection. The cost of repairs and liability would also depend on further investigations.

He was unsure if the steel box girder design of the bridge - still recognised today as being ``fairly innovative'' - would require it to remain closed during repairs.

A UiE spokesman said the oil platform had continued on its journey to the Captain field. It is due there on Wednesday.

It had suffered only ``minor damage'', which was expected to be rectified offshore within a couple of days.