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Kevin James scored his first goal of the season with perfect timing as Gillingham maintained their reputation as Millwall's bogey side.
Just as Sunday's cracking derby at the New Den was heading for a draw, up popped James with a spectacular last minute winner.
The goal not only gave Gillingham a third away win in the League but left Millwall without a win in 10 League games against their arch rivals.
Gillingham were much the better side in the first half and deservedly took the lead through player-manager Andy Hessenthaler whose shot deflected off Darren Ward in the 21st minute.
Millwall improved in the second half when they drew level through Nick Chadwick in the 55th.
A draw looked certain until James made his dramatic intervention with a 25 yard drive.
Attendance:12,084
Gillingham supporters: 1,248.