Initial bids were due by last Friday for National Grid’s gas distribution arm, which supplies millions of homes.
Foreign bidders are said to be eyeing the business, which analysts believe could be worth £11 billion.
Justin Bowden, GMB National Secretary for energy, said: “Prime Minister Theresa May should intervene and put this sale on hold until the review of the Enterprise Act is completed and there are proper safeguards in place.
“If it was good enough in her opinion to delay the go-ahead to Hinkley Point, then it is certainly good enough to put the brakes on here.
“This is not a cash cow but a key piece of infrastructure, vital to our economy.
“Just days after the government promised to review the Enterprise Act and introduce some long-overdue checks and balances into the sale of our key assets, yet another example comes down the line of fundamental UK infrastructure being flogged off to the highest foreign bidder.”