More than 4,000 U.K. contract workers downed tools at energy plants Tuesday in support of sacked workers at Total SA's (TOT) 200,000-barrels-a-day Lindsey oil refinery, in a dispute that has cost the French oil major EUR100 million.

At least 8% of contractors in the U.K.'s engineering construction industry walked out of more than a dozen sites Tuesday as strikes swept across the country, but the number of strikers is likely to be even higher because some companies haven't reported the extent of the walkouts at their facilities.

Total said Friday that its contractors had issued dismissal notices to workers on a hydro-desulfurization project at Lindsey. The contractors offered a deadline of Monday afternoon for workers to reapply for their jobs following a week of unofficial strike action. Jacobs Engineering Group Inc., the main contract company involved, has declined to comment.

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

The Lindsey dispute has become a flashpoint for skilled construction workers, sensitive to a slowdown in jobs and the use of foreign labor. The latest strikes - which are unofficial because they haven't been called by the unions or gone through the U.K.'s legal requirements for industrial action - are an escalation from several rounds of labor strife that erupted earlier this year and could widen in the coming days.

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

"We've been faced with significant delays and this project is on thin ice," said Total spokesman Iain Hutchison. "We've been disappointed by the poor execution of the project ... we want all issues to be resolved in order to get the project back on track and completed as soon as possible," he said.

Unite union spokesman Ciaran Naidoo added: "At the top of the agenda is obviously the reinstatement of the workers who have lost their jobs at Lindsey as soon as possible."

Despite the talks, formal negotiations through the arbitration agency ACAS have yet to be planned, an ACAS spokeswoman said.

Thousands of 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.

Other facilities affected by strikes included: Sellafield nuclear power station, 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, 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.

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