PARIS, Jan 27 (Reuters) - Trade unions at French utility EDF will launch legal challenges aimed at stopping the French government's move to cap electricity bills for consumers, they said on Thursday.

The government, which owns 84% of EDF shares, ordered the company on Jan. 14 to sell more cheap nuclear power to rivals to limit a rise in electricity bills to 4%, a policy that EDF said would cost it billions of euros.

Four unions will launch a legal challenge against the way the French Commission for the Regulation of Energy calculated the expected rise in power prices that should have taken place on Feb. 1.

They will also launch a second legal challenge, this time against the government's decision to force EDF to sell more power to rivals below market prices.

"We cannot let this company be torn to pieces, nor it be the last stand against a rise in power prices that is structural," they said in a joint statement.

Shares in EDF have lost 22% of their value since the beginning of the year.

Three months ahead of a presidential election, President Emmanuel Macron's government is facing mounting public pressure over the rising cost of living.

EDF has said the government's move could cost it up to 8.4 billion euros ($9.4 billion).

($1 = 0.8976 euros) (Writing by Gwladys Fouche Editing by Mark Potter)