BEIJING (Reuters) - China’s environment ministry issued a stern warning on Saturday to companies across heavy industry not to flout the nation’s tough emission rules -- a move seen as quashing speculation that the 2018 winter anti-smog campaign will be more lenient.

The warning comes after the Ministry of Ecology and Environment (MEE) on Thursday dropped blanket production cuts on heavy industry across northern China in its final winter anti-pollution drive and allowed local authorities to adopt flexible measures based on regional emission levels.

Shanghai benchmark rebar prices plunged nearly 5 percent this week as investors speculated that production curbs would be relaxed further this year. [IRONORE/]

“Pollution emitters must not take chances (on the new plan)... They will still be shut down or be ordered to enforce capacity cuts if they exceed emission standards,” said Liu Youbin, MEE spokesman, at a briefing.

Liu said improved air quality last winter showed production restrictions on heavy industry, from steel mills to coke plants, helped reduce toxic air that blankets the colder northern regions during winter when households crank up heating, mainly fueled by coal.

China will continue to carry out the curbs this year, but with more efficient measures to ensure blue skies, he said.

The yearly average concentrations of particulate matters, known as PM2.5, dropped by 35.6 percent in 2017 to 58 micrograms per cubic meter.

Still, forecasts of warmer temperatures, lower rainfall and wind in the north compared with last year may increase smog this winter, he said.

The government has said it aims to reduce average PM2.5 reading by around 3 percent this winter from the level last year.

Also on Saturday, Jincheng in Shanxi province became the latest city to issue its winter anti-pollution plan, telling coke makers to shut 30 percent of capacity from Oct. 1 and steel mills to cut output from Nov. 15.