A spokesman later said the additional power had been received.
Brian Strutton, national officer of the GMB union, said: "We now have the bonkers position where National Grid is using consumers' money to pay firms to stop work in order to avoid blackouts."
Shadow energy secretary Lisa Nandy said the government's "chopping and changing" with energy policy had created an "energy security crisis".
She said: "David Cameron should step in and end the policy vacuum to get new power stations built as quickly as possible."
