The European Commission has suspended all emissions trading after hackers broke into systems, stealing credits worth millions of euros.
The Emissions Trading System (ETS) is designed to cut pollution by making firms buy and sell carbon allowances, using national trading scheme registries.
However, following a spate of security breaches, the Commission says transfers at all member state registries will be suspended until at least 26 January.
Around EUR7 million in allowances have been reported stolen in the Czech Republic with accounts also hacked in Austria, Estonia, Greece and Poland as part of "recurring security breaches" over the last two months.
The EC says it has been working with national registries to improve security since a breach early last year and that "the incidents over the last weeks have underlined the urgent need for all registries to ensure that these measures are speedily implemented".