Brown's ruling Labour Party is widely tipped to lose a general election to the main opposition Conservatives that must be held by the middle of next year.
Darzi, appointed in 2007 by Brown as part of the prime minister's planned broad-based government, led a year-long review of the National Health Service, publishing a report last year.
"I am deeply grateful to you for having given me the privilege of serving in your Government, the freedom to make tough but important decisions, and the support to make change happen," Darzi said in his resignation letter to Brown and released by Downing Street.
"As you know, I have maintained my busy clinical practice and research contributions during my time as a Minister.
"The time has now come for me to return to care for my patients, lead my academic department, and continue my research on a full time basis."
In a statement, Downing Street said the prime minister had thanked him for an "outstanding contribution" as a health minister.
He would take up a new role as health and life sciences ambassador, the statement said.
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Darzi, appointed in 2007 by Brown as part of the prime minister's planned broad-based government, led a year-long review of the National Health Service, publishing a report last year.
"I am deeply grateful to you for having given me the privilege of serving in your Government, the freedom to make tough but important decisions, and the support to make change happen," Darzi said in his resignation letter to Brown and released by Downing Street.
"As you know, I have maintained my busy clinical practice and research contributions during my time as a Minister.
"The time has now come for me to return to care for my patients, lead my academic department, and continue my research on a full time basis."
In a statement, Downing Street said the prime minister had thanked him for an "outstanding contribution" as a health minister.
He would take up a new role as health and life sciences ambassador, the statement said.
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