Cameron was on his way back from visiting a farmer near his weekend home in the Oxfordshire countryside last month when he heard bleating and spotted a ewe that had got stuck in the mud after following her two lambs.
In jeans and rubber boots, Cameron waded into the swamp, followed by his two bodyguards, wrestled the sodden sheep and hoisted it onto safe ground, the Sun newspaper reported.
"When I got there, David (Cameron) was in the swamp, waist-deep in mud, along with the two police, who had all gone in there to help drag this sheep out," farmer Julian Tustian told the Daily Telegraph.
"He was brilliant, pulling, pushing and shoving. He was covered in mud, he looked a mess."
The ewe, which has since been nicknamed Swampy, has fully recovered from the ordeal, which unfolded on the evening of March 1, Tustian said. The lambs drowned.
The story of the lost sheep's messy rescue could have passed for an April fool joke but a Downing Street spokeswoman confirmed it had taken place.
Twitter users mocked what they labeled Cameron's "ewe turn", describing it as an unusual show of compassion from a prime minister imposing harsh welfare cuts and talking tough on immigration.
"Are you sure it was not the over way round? The ewe saved #Cameron from the swamp he's in?" wrote one user calling himself @Brianma68.
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