By Kate Abnett

BRUSSELS (Reuters) -Spain and Portugal have asked the European Union to step in to push for more power interconnectors with France, after a massive power outage hit the Iberian Peninsula last month, a letter seen by Reuters showed.

Spain and Portugal have limited power linkages to the rest of Europe and have said France has held up new interconnection projects that they say could help prevent disruptions like the unprecedented power outage that hit most of the Iberian Peninsula.

Works to strengthen an existing interconnector between France and Spain are expected to wrap up this year, while a new underwater power line spanning the Bay of Biscay is set to be completed by 2028.

In the letter to EU energy commissioner Dan Jorgensen, sent on Wednesday and seen by Reuters, Spain and Portugal's governments urged Brussels to step in to ensure new interconnection projects move ahead.

"A firm political and financial commitment is needed, at all levels, in order to ensure the swift and effective integration of the Iberian Peninsula into the EU energy system," said the letter, signed by Spanish energy minister Sara Aagesen and Portuguese energy minister Maria da Graca Carvalho.

"Spain and Portugal propose a ministerial meeting during this year in which, together with France and the Commission, we can agree on a roadmap with specific milestones and steps to be taken," the letter said.

A European Commission spokesperson confirmed it had received the letter and was in touch with the governments.

A spokesperson for France's energy minister did not immediately respond to a request for comment on the letter.

French grid operator RTE has studied the feasibility of building two additional interconnections with Spain over the Pyrenees in its multi-annual planning document published earlier this year.

RTE's planning report said it would expect the EU to contribute financing to any such upgrades, given the goal would be increased interconnection to Spain, "with the beneficiaries being located outside France."

France produces most of its power from nuclear plants, while Iberia uses a bigger share of renewable sources, whose fluctuating generation increases the need for flexibility in the power grid.

Iberia lags below the EU's target for countries to connect 15% of their electricity capacity to neighbouring countries by 2030 - with Iberia's share stuck at just 3%.

Spain and Portugal have argued this is driving up prices, and hampering their power grids' ability to respond to disruptions. Interconnectors can help stabilise energy grids by allowing power to flow between countries to respond quickly to supply and demand fluctuations.

"Accelerating the completion of electricity interconnections with the Iberian Peninsula must be placed among the highest priorities," the letter said.

Power outages of the magnitude seen in Spain and Portugal last month are rare in Europe. The blackout caused massive disruptions, grounding planes and forcing hospitals to suspend routine operations. The EU is investigating its cause.

A spokesperson for Spain's energy ministry and a spokesperson for Portugal's energy ministry each confirmed their ministers signed the letter.

(Reporting by Kate Abnett; additional reporting by Aislinn Laing, America Hernandez and Sérgio Gonçalves. Editing by Jane Merriman)