This is only the second time that divers from the company have been called on to undertake this type of repair work inland, and it is the first time that British Waterways has used the technique on its canals and navigable rivers.

As British Waterways Senior Project Engineer Kevin Kirkland explained, using the grout mattress eliminates the need for costly and disruptive pile driving.

"Normally, repairing a weir of this size means installing a cofferdam, which is formed by forcing big sheet piles around the existing structure and filling it with concrete. The cost to do this would have been around £1.5 million and we have four weirs on the River Severn requiring repairs", Kirkland explained.

"By installing a grout mattress, we are not only saving around £1 million per weir, we are also minimising the disruption to wildlife because the majority of work is carried out underwater, avoiding the need to dewater parts of the river."

Although working underwater keeps weather disruption to a minimum, unseasonably high rain levels in May put back the start date and could hamper work if they continue. The danger of divers being swept over the weir increases when the river is high and work may have to be halted from time to time.

It is estimated that repairs to the weir will take 16 weeks, although the final completion date is dependent on the amount of rainfall that occurs.

British Waterways says work must be completed by the end of September, however, as this is when fish migrations begin.

British Waterways is also taking care to ensure the protection of aquatic species during and after repair work - fish passes, tubelike structures which allow the fish to swim through rather than passing over the weir, are being repaired and specially designed eel boards are also being installed.

Preparation work for the repairs to Lincomb Weir is already complete. A pontoon has been constructed to allow the excavator access to the river and a local landlord has allowed access to British Waterways, Morrison and Sea Technical Services.

The first stage of the project, which is expected to take around four weeks, will see the mattress lowered into place and secured with weights and straps. Once it is securely in position, the mattress will be filled with grout to provide a solid waterproof structure at the front face of the weir, a similar technique to that used to protect the piers for the second Severn crossing bridge.

Once the grout mattress is in place, phase two will see a new concrete crest beam constructed at the top of the weir, to which the grout mattress will be permanently attached. The final phase will be to install new concrete slabs at the bottom or 'toe' of the weir.

This is the first time that the weir has undergone major restoration work, although it was extended by 3.6m during the 1930s to protect against further scour damage, which is caused by the force of the water eroding the sandstone structure.