Labour surged from bottom of the poll to the top when it snatched a surprise council by-election breakthrough.
Candidate Brian Oosthuysen won Gloucestershire's Rodborough division by a margin of three over the Tories, with a votes share surge of nearly 20% for the party since the last county contests in 2009.
This is particularly encouraging for Labour's chiefs since it trailed a poor fourth last time.
Rodborough is part of marginal Stroud constituency, lost to the Tories at the General Election.
The county result is a disappointment for both coalition Government parties. The Liberal Democrats missed taking the seat by 135 votes in 2009.
The Tories defended Stroud District's Amberley and Woodchester seat in the only other English by-election this week.
All three major parties increased their vote shares in the absence of the Greens but Labour's rise - at more than 10% - was the largest.
A further by-election result is due later today from Scotland's Angus Council.
Angus Council - Carnoustie and District: Counting today. (May 2007 - Three seats SNP 1837, Lab 1160, C 863, Lib Dem 748, SNP 639, Solidarity 62. Elected SNP 2, Lab 1).
Gloucestershire County - Rodborough: Lab 793, C 790, Lib Dem 660, Green 260. (June 2009 - Three seats C 1196, Lib Dem 1061, Green 716, Lab 415). Lab gain from C. Swing 11.5% C to Lab.
Stroud District - Amberley and Woodchester: C 366, Lab 177, Lib Dem 124. (May 2008 - C 452, Green 151, Lab 131, Lib Dem 98). C hold. Swing 5.1% C to Lab.