The two companies say they will file a petition no later than Dec. 17 but haven't indicated on what grounds they are making the request.
Under Washington State law, the UTC has 20 days to consider the petition, otherwise it is deemed to be denied.
If it reconsiders its decision, the UTC can modify the prior order or take any actions it deems appropriate including extending deliberations.
Washington State regulators said they would not allow Ontario's largest utility to buy Avista for fear the provincial government, which owns 47 per cent of Hydro One's shares, might meddle in Avista's operations.
Hydro One's shares have risen since the order because the deal, announced in July 2017, would have eroded earnings per share and weakened Hydro One's balance sheet, according to analysts.