Voters got the "wrong winner" in councils across the Midlands after May's local elections, a report claimed.
In a number of authorities, including Birmingham, the party with the most votes failed to win the most seats.
The findings were published by the Electoral Reform Society, which called for reforms to Britain's voting system.
But the calls were rejected by Birmingham MP Sion Simon (Lab Erdington), who said changes could help fringe parties such as the BNP.
In Birmingham, the Conservatives won 26.1 per cent of the vote while Labour won 31.5 per cent.
But the Conservatives won 17 seats, and Labour won only 15 - helping the Tories to retain control of the council in a coalition with the Lib Dems.
An Electoral Reform Society report, called the Great Local Vote Swindle, said: "There was a 'wrong winner', in that the party that won most seats had fewer votes than another party."
Sometimes the system worked to Labour's advantage. In Nuneaton and Bedworth, in Warwickshire, the Conservatives won 51.4 per cent of the vote and eight seats, while Labour won nine seats with just 38.1 per cent of the vote.
The report said: "In Nuneaton and Bedworth there was an extremely unfair result... Labour has a majority of two on the council despite having lost the popular vote."
In other councils the right party was declared the winner, but the losing party received far fewer seats than it should have done, the society said.
For example, in Tamworth the Tories won ten seats with 50.9 per cent of the vote but Labour, which received 41.5 per cent of the vote, had to settle for just two seats.
The society called on the Government to end some of the anomalies by replacing the existing "first-past-the-post" electoral system with proportional representation.
Ken Ritchie, of the Electoral Reform Society said: "If a voting system cannot even delivery the outcome that people vote for then it is clearly failing."