Boris Johnson's plan for the Treasury to publish the costings of key Labour policies was blocked last night after John McDonnell "kicked off", the Mirror understands.
The Tories were left fuming when Sir Mark Sedwill, the country's top civil servant, stopped the plan to use tax-payer funded resources on the eve of the election.
A Labour source told the Mirror: "This is an embarrassing slap in the face for Tory ministers, who have been caught red-handed attempting to blatantly use the civil service for party political purposes in an unprecedented way".
Chancellor Sajid Javid boasted to the Cabinet earlier in the day that the costings of up to a dozen flagship Labour policies were about to be released.
But when Treasury chief Sir Tom Scholar made a courtesy call to Mr McDonnell to inform him of the publication he was "taken to task".
The shadow chancellor told him Labour had not confirmed what would be in its manifesto so the analysis would be pure speculation.
"I'm happy for anyone to examine our policies, but to do this hours before a general election campaign is I think an abuse of power," he later said.
Sir Mark then intervened to block publication, prompting the fury of Tory ministers.
A Treasury insider said: "Ultimately this is a question for him but this is not an unusual or unorthodox practice. It has been going on since the time of Gordon Brown."
But although the Treasury analysis of Opposition spending pledges took place ahead of the 2010 and 2015 elections, former Chancellor Philip Hammond decided not to ask for it at the 2017 election.
Civil servants are banned from doing any work that could call into question their impartiality in the period known as "purdah" ahead of a general election campaign.
