(Adds Hungarian reaction, paragraphs 7-8)

By Shadia Nasralla

VIENNA, Jan 22 (Reuters) - Austria is planning to sue the European Commission for allowing Hungary to expand its Paks atomic plant, it said on Monday, not viewing nuclear energy as the way to combat climate change or as being in the common European interest.

The country, which shares a border with Hungary, prides itself on supporting environmentally sound energy. It has for decades opposed nuclear power, which triggers huge disagreements about cleanliness, safety, and renewability.

The anti-nuclear position was reiterated in a coalition agreement struck last month between Chancellor Sebastian Kurz's conservatives and the far-right Freedom Party.

"We in the government have agreed that there are sufficient reasons to sue (the Commission)," a spokesman for Austrian Sustainability Minister Elisabeth Koestinger said.

"EU assistance is only permissible when it is built on common interest. For us, nuclear energy is neither a sustainable form of energy supply, nor is it an answer to climate change."

EU state aid regulators approved last March Hungary's plan to build two new reactors at its Paks nuclear site with the help of Russia's Rosatom, saying Hungarian authorities had agreed to several measures to ensure fair competition.

The two new reactors will double the plant's nominal capacity of 2,000 megawatts. Hungary aims to start construction on the reactors this year, with the first facility expected set for completion in 2025.

"The Paks nuclear plant is the guarantee for providing a cheap, reliable and safe supply of electricity to Hungarian people and businesses," Hungarian Prime Minister Viktor Orban's office said in a statement.

"Therefore, the Hungarian government will stick to its plan to ensure the maintenance of capacity at the Paks plant," it said, adding that the lawsuit would not affect work on the project.

The deadline for filing a suit to challenge the executive EU Commission's decision at the European Court of Justice is Feb. 25, the spokesman for Austria's Koestinger said.

In a majority of such complex cases, the European Court of Justice in Luxembourg has found in favour of the Commission.

Austria launched a similar legal action against the European Commission in 2015 over its backing of British plans for a 16 billion pound ($22.24 billion) development of the Hinkley Point nuclear power plant. ($1 = 0.7195 pounds) (Reporting By Shadia Nasralla; additional reporting by Krisztina Than in Budapest; Editing by Gareth Jones/Jeremy Gaunt)