MPs have backed a bill calling for a change in the voting system for elections.

In a major shock, the Elections (Proportional Representation) Bill was backed by 138 votes to 136.

The bill, in the name of the Lib Dem MP Sarah Olney, calls for the current first past the post system to be replaced by a form of PR.

She said: “This is a historic day in the fight for fairer votes and I am grateful to all the MPs who backed it.

“Trust in our political system is broken following years of the Conservative Party riding roughshod over standards in public life.

“Fixing our broken electoral system, introducing fair votes, and making sure everyone’s voice matters is the best way to rebuild this trust.

“Today, as we have done for a century, Liberal Democrats are leading the fight for fair votes and making sure that no one can be ignored in our democracy.

“The government must now listen to the will of the House, make time for the legislation and make fairer votes a reality and we will be holding their feet to the fire to make this happen.”

However, a spokesman for the prime minister said: “There are no plans for proportional representation or reforms to the voting system.”

Unless the government makes time for the bill at second reading, which it appears unlikely to do, the bill will fall.

A breakdown showing how MPs voted this afternoon showed Labour is split over the issue, with 59 of the party’s MPs voting for it and 50 voting against.

During the Commons debate on the bill, Tory MP Lewis Cockington suggested British voters may struggle to understand PR, despite it being used in other countries around the world.

He said: “It cannot be denied that voters would be confronted with a far more complicated system if any type of proportional representation were to be introduced.”

In a referendum on introducing the alternative vote method of PR in 2011, 68% of voters supported sticking with the current system.