Simon Clarke tweeted that his condition “prevents me being comfortable in some open spaces” and so as a result would not take part in the shots outside No 11 Downing Street.
Agoraphobia is commonly thought of as a fear of being in open spaces, but it is a more complex condition which also included being fearful of being in situation where escape might be difficult or that help wouldn't be available if things go wrong.
The Treasury ministerial team is ritually photographed together outside the famous black door while the chancellor holds up their Budget red box before the Commons fiscal speech.
The chief secretary to the Treasury is the most senior person in the department behind the chancellor.
The photograph has on occasion, including in March before Chancellor Rishi Sunak’s spring speech, been taken inside Downing Street.
But Mr Clarke, tweeting an indoor photo of him alongside Mr Sunak on Wednesday, explained he would be absent from the autumn photo as it is due to be posed in the street.
“Really looking forward to explaining the Budget and SR (Spending Review) alongside the chancellor,” he said on social media.
“I won’t be outside for the photos in Downing Street as I live with agoraphobia – which prevents me being comfortable in some open spaces – but will be busy in Parliament and out in the country over the coming days.
“Today is all about a major moment for the UK and we have an important story to tell about investment in our public services and infrastructure, economic recovery, levelling up and net zero – those are the issues I’m proud to be discussing today.”
Mr Clarke was promoted to Chief Secretary in Boris Johnson’s September reshuffle.
He had previously been a local government minister but resigned in 2020, citing personal reasons understood to relate to his family life.
Before his local government post, the Conservative MP for Middlesbrough South and East Cleveland had previously been exchequer secretary to the Treasury.