Matt Hancock acted unlawfully when his department did not reveal details of contracts it had signed during the Covid pandemic, a court has ruled.
A judge said the health secretary had "breached his legal obligation" by not publishing details within 30 days of contracts being signed. The public had a right to know where the "vast" amounts spent had gone and how contracts were awarded, he added.
The Department for Health and Social Care (DHSC) has struck deals worth hundreds of millions of pounds during the coronavirus pandemic.
Campaign group the Good Law Project and three MPs - Labour's Debbie Abrahams, Green Caroline Lucas and Lib Dem Layla Moran - took legal action against the department over its "wholesale failure" to disclose details of the contracts agreed.
Under the law, the government is required to publish a "contract award notice" within 30 days of the awarding any contracts for public goods or services worth more than £120,000.
The Good Law Project also claimed that the government breached its own transparency policy, which requires the publication of details of public contracts worth more than £10,000.
In his ruling, Mr Justice Chamberlain said: "There is now no dispute that, in a substantial number of cases, the secretary of state breached his legal obligation to publish contract award notices within 30 days of the award of contracts. There is also no dispute that the secretary of state failed to publish redacted contracts in accordance with the transparency policy."
The judge said the health secretary had spent "vast quantities" of public money on Covid-related goods and services during 2020.
"The public were entitled see who this money was going to, what it was being spent on and how the relevant contracts were awarded," he added.
He said this was important so that competitors of those awarded contracts could understand whether the obligations had been breached.
Bodies such as the National Audit Office, as well as Parliament and the public, to "scrutinise and ask questions about this expenditure".
Mr Justice Chamberlain acknowledged that the situation faced by the DHSC during the first few months of the pandemic had been "unprecedented".
He said it was "understandable that attention was focused on procuring what was thought necessary to save lives".
But he added that the DHSC's "historic failure" to publish details of contracts awarded during the pandemic was "an excuse, not a justification".