* Risks of shortages as closures loom

* France aims to reduce nuclear reliance

* Fresh reactor restart delays announced on Tuesday (Adds detail, analyst)

PARIS, Nov 7 (Reuters) - France risks power supply shortages after 2020 if it moves ahead with plans to simultaneously shut down four 40-year-old nuclear reactors and all its coal-fired plants, grid operator RTE said on Tuesday.

France aims to phase out coal and by 2025 to reduce nuclear power in its energy mix to 50 percent from 75 percent by shutting some reactors and increasing its use of renewable energy.

President Emmanuel Macron had promised to close all coal-fired plants by 2020, switching off some 3 gigawatt in capacity.

Beyond 2025, France will need to add about 11 GW in capacity using renewables and gas fired plants, RTE said in its 2017-2035 Electricity Outlook, citing French plans to shut down about 24 of the country’s 58 nuclear reactors.

That could increase and even double France’s carbon dioxide (CO2) power generation emissions, RTE said.

Ecology Minister Nicolas Hulot is expected to address the topic of France’s long-term energy mix at a news conference after a ministerial meeting later on Tuesday.

MORE RESTARTS DELAYED

As for this winter, RTE said electricity demand was expected to be stable, although unplanned nuclear reactor outages and a prolonged cold spell could squeeze supply.

It said it was keeping a watchful eye on several nuclear reactors operated by utility EDF that are currently offline but due to return to service this month.

“The forecast for winter 2017-2018 is stable compared with last year, with electricity demand estimated at 221.5 terrawatt-hour in the period from mid-November to the end of March,” RTE said.

In the event of a cold spell and prolonged nuclear outages, RTE said it could be forced to take exceptional measures to guarantee security of supply.

European and French power prices have hit new highs in recent weeks over worries that France could face another tight winter supply situation due to reactor restart delays and review ordered by nuclear regulator ASN.

Fresh delays to the restart of eight EDF reactors boosted European prompt power prices early on Tuesday. Most of the postponed restarts will affect this month and December.

The RTE said it would have 2.8 GW more production capacity compared with the same period a year ago and could also count on 8.1 GW of import capacity from neighbours, a 1 GW increase compared with last year.

Traders said while the situation was better than a year ago there were still concerns that the delays could be extended.

“French nuclear availability will once more be a key driver for European gas, power, coal and carbon prices this winter,” said Wayne Bryan, analyst at consultancy Alfa Energy. “Each announcement regarding restarts will move markets.” (Additional reporting by Nina Chestney; editing by Sudip Kar-Gupta and Jason Neely)