ANAS Sarwar has been urged to clarify whether he backs a plan to apply a "nuclear tax" to Scots with bills set to go up due to the rising cost of an English nuclear plant.
Energy Security Secretary Ed Miliband has confirmed the Sizewell-C plant will cost £38 billion, nearly double the previous estimate of £20bn.
Miliband snuck out a statement hours before Parliament was due to go into a six-week summer recess, admitting energy bill payers would face a decade-long levy as a result of the price hike.
This is despite Labour promising ahead of the General Election that their flagship GB Energy policy would save people £300 a year on their energy bills. In actual fact, bills are on average 10% higher than they were this time last year.
It also comes after Chancellor Rachel Reeves cut £2.5 billion from GB Energy to fund English nuclear reactors at the Spending Review.
An average of £1 will be added to each household’s energy bills per month from autumn, over the duration of the nuclear site's construction phase.
The letter says: "These English nuclear power plants are extortionate, but the bombshell announcement that your Labour Government slipped out was that they are to be funded by a decade-long levy on Scottish energy bills.
"To be absolutely clear: in energy rich Scotland, Scots will pay more money on their energy bills to pay for English nuclear power plants.
"If Ed Miliband's stated figures are correct, it could see Scots foot a £300m bill for a nuclear power plant south of the border – a 'nuclear tax' on every Scottish energy bill. A grave injustice and a far cry for your party's pledge to cut energy bills which have soared by £150 on your watch.
“Any politician from Scotland worth their salt will call this out for what it is - completely absurd. Yet far from that being the case, it seems Scottish Labour is happy to cheer on investment in England at the expense of Scottish industry and Scottish bill payers.
"You and your party stand squarely behind these extortionate white elephants despite Scotland's energy future being bound in the success of renewable energy projects, carbon capture and closer links with Europe.
“Your continued silence on this matter begs the question whether you support these plans that will punish Scottish bill payers - if not, when will you speak out?"
Sarwar has previously urged John Swinney to end the Scottish Government's "ideological block" on nuclear power.
The UK Government announced on Tuesday that it had signed a deal with a group of investors on Sizewell-C.
The Government will become the biggest equity shareholder in the project with a 44.9% stake.
New investors include Canadian investment fund La Caisse with 20%, British Gas owner Centrica with 15%, and Amber Infrastructure with an initial 7.6%.
Scottish Labour have been approached for comment.
