LONDON, May 20 (Reuters) - Britain may impose anti-dumping duties of up to 29% on aluminium extrusions from China to protect domestic producers, a trade agency said on Friday.

Aluminium extrusions - widely used in the transport, construction and electronics industries - are being dumped in Britain at lower prices than they are sold in China, the Trade Remedies Authority said in a interim report.

"The TRA determined that there is already damage to the UK industry, having found clear evidence of price undercutting, indicating that UK businesses are struggling to compete with the dumped imports," a statement said.

Provisional measures will be imposed as the TRA completes its investigation, requiring Chinese companies exporting to Britain to provide a bank guarantee beginning on May 28, it added.

Duties ranging from 7.3% to 29.1% were recommended, depending on the company and the level of dumping margin, the interim report said.

Three companies were named - Press Metal International Group, Shandong Nanshan (600219.SS) and Haomei Group - plus there were categories for other co-operating and non-cooperating exporters.

Press Metal International is a Chinese unit of Malaysia's Press Metal (PMET.KL).

The TRA was established after Britain left the European Union to investigate unfair trade practices and measures.

The aluminium extrusion investigation is the first one resulting from a British industry claiming unfair trade practices, the TRA said.

One major firm that produces aluminium extrusions in Britain is Norway's Norsk Hydro