The Hunterston unit, which can produce enough electricity to power more than 800,000 homes, has been offline since March when cracks were found in its core during a routine inspection.
UBS’s Sam Arie called the update “significantly negative”, with 2019 “not looking to get any easier than 2019”.
“The case for a reset or wider rethink of pricing strategy at British Gas may start to build,” he added.