Labour has gained a key seat at Trafford Borough, Greater Manchester, one of only two Tory-controlled metropolitan authorities.
The win opens the way to a tense battle in May's main council polls.
The Broadheath by-election triumph by candidate Helen Boyle cuts the Conservatives' town hall majority to just three.
If the swing was repeated across the borough in May this would leave Tories with a lead of one.
There was a small shift to Conservatives since the 2012 elections when Labour won another seat in the ward.
However the swing from May 2010, on the same day as the general election, was more than 8% to Labour.
Broadheath is next door to Wythenshawe and Sale borEast where a Commons by-election is pending.
Trafford Borough - Broadheath: Lab 1377, C 1258, Ukip 234, Lib Dem 150, Green 67. (May 2012 - Lab 1662, C 1397, Ukip 221, Green 156, Lib Dem 152). Lab gain from C. Swing 1.8% Lab to C.
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