Simon Clarke said the condition – which can include a fear of being in situations where it might be difficult to escape – “prevents me being comfortable in some open spaces”.
As a result, he did not stand alongside the chancellor, Rishi Sunak, and other Treasury ministers in the photos taken outside No 11.
Mr Clarke instead tweeted an indoor photo of him alongside Mr Sunak on Wednesday.
He said: “Really looking forward to explaining the Budget and SR [spending review] alongside the Chancellor.
“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.”
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The NHS website describes agoraphobia as a “fear of being in situations where escape might be difficult or that help wouldn't be available if things go wrong”.
The health service guidance says people often wrongly assume that agoraphobia is simply a fear of open spaces, but that it is actually a “more complex condition”, with fears associated with the condition including travelling on public transport, visiting a shopping centre or leaving home.
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.
Additional reporting by PAInternet Explorer Channel Network