Unofficial strike action swept across the U.K Tuesday where more than 4,000 contract workers downed tools at energy plants in support of sacked workers at Total SA's (TOT) 200,000-barrels-a-day Lindsey oil refinery.

At least 8% of contractors, or 4,000 workers, in the U.K.'s engineering construction industry are now on strike at more than a dozen sites across the country. However, the number of strikers is likely to be even higher because some companies haven't reported the extent of the walkouts at their facilities.

Talks between the Lindsey contractors and unions have started in London over how to facilitate the return of contract workers to a construction project at Lindsey. Total said it is "actively encouraging" the talks.

However, a spokeswoman for conciliation agency ACAS confirmed there will be no official joint talks taking place through them today.

Spokesman for the union Unite Ciaran Naidoo said negotiations will review a working practice agreement signed by both parties through the Engineering and Construction Industry Association.

"At the top of the agenda is obviously the reinstatement of the workers who have lost their jobs at Lindsey as soon as possible," Naidoo said.

Total said delays, under-performance and low productivity at the project have already cost in the region of an additional EUR100 million.

Friday, Total fired 647 contract workers at Lindsey following unofficial industrial action, and offered a deadline of Monday afternoon to contractors to reapply for their jobs. Total said it will be the end of the week before its contractors can say how many workers will return to work.

"There is no question of a reduction in pay or dilution of existing terms and conditions," it said in its statement Tuesday.

Around 4,000 workers extended unofficial strikes in sympathy with the Lindsey contractors Tuesday, although operations weren't affected at any of the sites where walkouts were reported.

These protesters included 900 workers at the Sellafield nuclear power station who joined the action Tuesday but are expected to return to work Wednesday.

Other facilities affected by strikes Tuesday included: ConocoPhillips' (COP) 221,000-barrels-a-day Humber refinery, Royal Dutch Shell PLC's (RDSB.LN) 240,000-barrels-a-day Stanlow refinery, RWE AG's (RWE.XE) Aberthaw and Didcot A power stations, Drax Group PLC's (DRX.LN) Drax power station, the U.K.'s largest coal-fired power plant, the Dragon liquefied natural gas terminal in Wales, BP PLC's (BP) Saltend chemical plant and at Ensus Group's biofuel plant under construction on Teesside.

"Unless action is taken fast to resolve this dispute, deadlock will continue, sympathy strikes will escalate and the U.K.'s energy supply will be under serious threat," said Graham Botwright, managing partner at industrial relations consultancy The Gap Partnership.

Botwright said progressive talks will only gain ground if a new third-party arbiter is appointed, or even a new lead negotiator for each side. "Industrial action should always be the last resort. Once industrial action has been employed, the collective bodies lose much of their bargaining power," he said.

-By Angela Henshall, Dow Jones Newswires; (4420) 7842 9285; angela.henshall@dowjones.com (James Herron and Lananh Nguyen contributed to this story.)