The EU extended a freeze Thursday on trading in carbon credits ordered after hackers broke into national trading registries and stole and then sold millions of euros worth of polluting rights.
"Security comes before trading," said Maria Kokkonen, spokeswoman for European Union climate action commissioner Connie Hedegaard.
"It's going to take time," she told AFP, with only internal trading in the best-protected national registries targeted for resumption next week.
Originally, it was suspended for a week which would have meant restrictions being lifted as of Wednesday evening.
Less than half of the 27 EU states are deemed by the commission to have implemented adequate online security, although minimum standards have now been agreed by experts from around the bloc.
The European Union Emission Trading Scheme (ETS) is the largest multi-national, greenhouse gas emissions trading scheme in the world, with about 12,000 companies on the exchange.
Limits are placed on the amount of carbon dioxide companies may emit, and those who pollute less are free to sell them to companies that need more.
The system has frequently been attacked, with another halt ordered in trading in numerous countries last year after "phishing" emails tricked users into divulging their passwords and a VAT scam in 2008 and 2009 netting criminals five million euros.
Explore further: Cybercrime now 'number one' threat: Europol chief