The government has been accused of “sneaking out” changes to the way local elections are run, without consulting MPs or the public.

The change will impose the controversial ‘First Past the Post’ voting system on all elections for local mayors and police and crime commissioners (PCCs).

The system is already used for general elections, and has allowed British politicians to be elected with as little as 25% of the vote.

Opposition parties have responded by accusing the Conservatives of “rigging democracy in their favour” and taking away choice from voters.

The current supplementary voting system for mayoral and PCC elections allows voters to give their second preference – meaning parties need a wider base of support to win. If no candidate wins 50% of the votes, then second preferences are added in.

Announcing the change, Cabinet Office minister Chloe Smith said the new system is “fair and simple” and “allows voters to kick out the politicians who don’t deliver”.

Another minister, Luke Hall, claimed that the current system “confuses the public”.

But the Electoral Reform Society (ERS) today said that the move was an “attack on voters’ ability to cast their ballot and have their voice heard”.

Dr Jess Garland, the ERS’s director of policy and research, said that imposing Westminster’s "broken" First Past the Post voting system on mayoral and PCC elections was a "step backwards".

"This is a regressive move that would likely see significant positions handed to people without the support of a majority of the voters – undermining the legitimacy of those elected."

She added: “These proposals show yet again the government has little interest in improving democracy, but instead is using this bill to undermine our democracy and cast the views of voters on the scrap heap.”

Green Party MP Caroline Lucas told openDemocracy: “This measure, sneaked out during a Cabinet reshuffle, means the only elections in our country which were free of the discredited, outdated and broken voting system of First Past the Post’ are being pulled back into line.

“This has nothing to with strengthening accountability, as the government claims. It's about shoring up a bankrupt form of voting because it benefits the Tories. It takes away choice from voters because too often they have to choose a candidate they may not want just to stop the one they dislike the most from winning.

She added: “This is not how democracy should work, and it doesn't have the support of voters, but it's the way our democracy is heading under this Tory government.”

The Labour Party, which also criticised the announcement, accused the Conservatives of “rigging democracy in their favour instead of working to strengthen it”.

Cat Smith, the party’s shadow minister for democracy, said: “While the Welsh Labour government has taken radical steps to bring people into democracy and empower voters, the Conservative government refuse to take any action to improve our outdated elections and are focused solely on changing the rules to benefit the Conservative Party.”

The changes will apply to the election of council and "metro" mayors across England, and to Police and Crime Commissioners across both England and Wales.

The move comes as MPs debate the government’s Elections Bill, which critics see as a power grab by the Conservative Party.

Writing for openDemocracy, a former electoral commissioner condemned the “appalling” plans, which include restricting the independence of the UK’s election watchdog.

“It is difficult to express just how appalling this is,” said David Howarth. “Electoral Commissions, like the courts, do not exist to please elected politicians. They exist to protect free and fair elections, which they can’t do unless they are independent and free from the control of the ruling party.”