Energy companies will have to disclose the timing of gas field maintenance and nuclear shut-downs under new EU legislation about to be finalised and expected to start taking effect around the end of this year.
The law will not cover highly market-sensitive information on outages on oilfields, long a source of controversy among oil traders, although knowledge of when associated gas is shut down could provide a clue.
Nuclear shutdowns and gas field maintenance would typically trigger a disclosure obligation, said Lucas Bergkamp, a partner at Hunton & Williams in Brussels.
Another international law firm, Cleary Gottlieb, issued a note saying inside information would include anything linked to the capacity and use of facilities for production, storage, consumption or transmission of electricity or natural gas and use of LNG facilities.
That would include planned or unplanned disruptions.
REMIT (Regulation on Energy Market Integrity and Transparency) is a first step in the extension of traditional prohibitions of insider trading and market manipulation to physical commodities market, the note concluded.
Commission officials said they were working on the precise details of which data the energy firms would have to publish.
The data will have to be published six months after the first stage of the law is enforced.
The first step, making insider trading and market manipulation illegal, will be binding 20 days after publication in the EU Official Journal, expected over the coming weeks. The draft rules have already been widely publicised.
For the British wholesale power and gas markets, utility EDF has started publishing a nuclear outage table, but information on some other plants and on gas field maintenance can be hard to obtain.
The issue (for traders) is that everyone wants to know what the other one is doing, but no one wants to reveal their information. Regulation is a way to enforce this, so people won't have the choice, said a gas trader at a major European utility, who asked not to be identified.
He also raised one of the standard objections to new EU law.
Enforcing these rules on companies will cost quite a lot. Updating their systems, etc. requires quite a lot of work, he said.
Traders and energy companies in Germany took the view it would change little for a market regarded as a model of transparency and that the same rules should be applied elsewhere.
RWE Supply and Trading and E.ON Ruhrgas reveal on their websites production and outages in real time.
The EEX (European Energy Exchange) also has live information on installed capacity.
E.ON supports regulatory plans which serve to strengthen trust in energy markets, for example transparency guidelines, if they apply to the whole of Europe uniformly, an E.ON spokesman said.
Transparency is being handled very differently in different parts of Europe at the moment.
Commission officials said oil had been deliberately excluded from the regime, as it is regarded as more international than Europe's wholesale power and gas markets.
Oil is an international commodity. Enforcing transparency rules needs to be tackled at international, not European level, an official said.
Whereas gas and electricity are fundamentally European markets, as they need to be delivered by a network.
But the publication of supply outages that can have a big impact on physical oil markets, with knock-on implications for futures trade, has yet to be covered.
Major oil companies say they have internal barriers designed to prevent their trading departments benefiting from the knowledge of field operators about when oil supplies fluctuate because of outages.
BP and Royal Dutch Shell had no immediate comment on REMIT.
(Additional reporting by Karolin Schaps and Alex Lawler in London; editing by Rex Merrifield)