Labour is No help for Cameron

David Cameron is to take the unprecedented step of joining senior Labour politicians to campaign against the alternative vote, in the clearest sign yet that the No camp fears it could be defeated in next month's referendum.

In a surprise move, the Conservative Prime Minister will join forces with his arch rivals to warn against dropping first past the post for Westminster elections. Tory sources said preventing electoral reform would take precedence over campaigning in elections to English councils and the Welsh and Scottish parliaments. "It is all about AV – there is no question this is the PM's priority," said one adviser.

Public opinion about the issue of electoral reform is becoming more polarised as the 5 May polling day approaches. An exclusive ComRes/ IoS survey shows a lead for the No campaign, but increased support for both sides. Of those who intend to vote, 37 per cent said they would vote Yes, up 3 points since March; the No vote is up 6 points to 43 per cent.

However, with a fifth of those who intend to vote yet to make up their minds, both campaigns will step up their efforts this week. The outcome of the referendum is seen as critical to the stability of the coalition, with both Mr Cameron and Nick Clegg at risk of mutiny in their ranks if they lose.

The Labour Party is split on the issue, with the party's leader, Ed Miliband, supporting AV while more than 100 MPs, including several members of the Shadow Cabinet, oppose change. Party stalwarts, including the former deputy prime minister, Lord Prescott, and the ex-foreign secretary, Margaret Beckett, have been at the forefront of the No to AV campaign. It is understood that Lord Reid, who was a home secretary under Tony Blair, will be among those sharing a platform with Mr Cameron this week.

In a sign of growing tension between the two coalition parties, Mr Clegg used a speech in Sheffield yesterday to claim Conservatives in the No campaign were becoming "desperate".

Paddy Ashdown, a former Lib Dem leader accused George Osborne of mudslinging last night after the Chancellor claimed that the Electoral Reform Society will profit from a Yes vote.

Lord Ashdown, writing in The Observer, said: "This strategy stinks of the same odour which has surrounded politics recently.... George Osborne makes the case for change for us. He graphically shows why we need to change our politics."