Writing in the Daily Mail, Mr Johnson says the Prime Minister's 'petty' removal of the famous painting from her former study shows he misled the public when he praised her during his campaign.
The private boys' school in Berkshire - previously attended by famous names including Princes William and Harry, Boris Johnson, Eddie Redmayne and Bear Grylls - said it is "disappointed" about having to impose VAT on fees.
Boris is the endorsement they all want,' a former Cabinet Minister told me. 'He's not got directly involved in the contest yet. But he's watching. And if it's close, he could still turn out to be the kingmaker.
Robert Jenrick is not the only politician to have battled the bulge in recent years, with David Cameron, Boris Johnson and Tom Watson also having waged war on the scales.