GMB, the union for energy workers, accused National Grid of using the vital piece of UK infrastructure — which supplies gas to 11m British homes and businesses — as a "cash cow".
It comes a day after bidders from around the world submitted preliminary offers ahead of a deal which will be an early test of Prime Minister Theresa May's new terms for foreign investment in UK infrastructure.
"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," Justin Bowden, GMB National Secretary for energy, said.
"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."
Hong Kong billionaire Li Ka-shing, a Chinese consortium including Fosun and the state behemoth China Gas, a group led by the Canada Pension Plan Investment Board and another led by the Australian bank Macquarie are thought to be bidders.
Whitehall recently approved the controversial Hinkley Point C nuclear power plant but with revised conditions, after pushing back its decision due to security concerns over Chinese investment.