A British minister announced his unexpected resignation on Saturday over a story due to be published in a tabloid newspaper, in a blow to David Cameron's Conservative-led government.

It was the second setback to the prime minister on the eve of the Conservatives' final annual conference before the May general election, following the defection of a lawmaker to eurosceptic party UKIP earlier on Saturday.

"I have decided to resign as Minister for Civil Society having been notified of a story to be published in a Sunday newspaper," said Brooks Newmark, a Conservative member of parliament since 2005.

"I would like to appeal for the privacy of my family to be respected at this time. I remain a loyal supporter of this government as its long-term economic plan continues to deliver for the British people," Newmark said.

The Sunday Mirror story, expected to be published tomorrow, was reported to involve social media.

"The prime minister has accepted Brooks Newmark's resignation from his role as Minister for Civil Society. His replacement will be announced shortly," Cameron's office said in a statement.