Last week, the regulator struck down Hydro One's proposed acquisition of its U.S. peer, saying it would not adequately protect Avista or its customers from political and financial risks.

Under Washington law, the UTC has 20 days from the date of filing to act on the petition, failing which the petition is deemed to be denied, the companies said in a statement.

Hydro One offered C$6.7 billion ($5.02 billion) to take over Avista in 2017 as it sought to expand into the U.S. Northwest.

(Reporting by John Benny in Bengaluru)