London: Britain's political leaders prepared for a final frenetic 48 hours of campaigning from Tuesday ahead of Britain's knife-edge general election on Thursday.

Conservative Prime Minister David Cameron and Labour opposition leader Ed Miliband said that voters faced a stark choice two days from the opening of polling stations.

With polls showing the two main parties neck-and-neck and unlikely to win a majority in Thursday's vote, power will likely hinge on the performance of smaller parties such as the Scottish National Party (SNP) and the Liberal Democrats.

Cameron will urge voters to back his party and avoid the ‘instability’ he argued would result if a single party did not win a strong mandate.

‘Back-room deals. Bribes. Ransom notes. Chaos. Not just for the week after the election. But for five long years,’ he said in pre-released remarks, in an appeal to those considering voting for smaller parties.

Miliband has described the election as a ‘clash of two visions’ and tried to hang the remaining election battle on contrasting plans for the state-funded National Health Service.

‘In the final few days of this general election, the future of the NHS is at risk in the way it hasn't been for a generation,’ Miliband said on Monday.

The latest BBC poll of polls out Saturday gave the centre-right Conservatives 34 percent and centre-left Labour 33 percent.

The populist UK Independence Party were on 14 percent, the centrist Liberal Democrats eight percent, the left-wing Greens on five percent and others on six percent.

These figures would leave both major parties well short of winning the 326 seats needed for an absolute majority in parliament's lower House of Commons -- meaning lengthy negotiations to form alliances could follow Thursday's vote.

The unpredictability of the election was underlined when a row broke out between the Conservative and the Liberal Democrats who have been their junior coalition partners since 2010.

- Angry hecklers –

The spat occurred after Liberal Democrat campaign spokesman Paul Scriven claimed that Cameron had privately admitted his party won't win a majority.

‘If David Cameron is going to lie when he knows in his heart of hearts his private polls are showing that the British people are likely to give no party an overall majority, then so be it,’ Scriven told the BBC.

‘People are not stupid. The opinion polls show what's happening.’

The Conservative party denied that Cameron had admitted he might not win enough seats to govern alone.

‘The Prime Minister has repeatedly made clear we only need to win 23 seats to get a majority and he is totally focused on winning that majority,’ a spokesman said.

The BBC later reported that senior figures in the Labour party were considering the possibility of forming a minority coalition with the Liberal Democrats.

The Liberal Democrat party, whose leader Nick Clegg has been Deputy Prime Minister under Cameron, has repeatedly said it will speak first to the bloc that wins the most number of seats in any post-election negotiations.

Though SNP leader Nicola Sturgeon has offered to help Miliband become prime minister, the Labour leader has ruled out a deal with the secessionist party.

The SNP has surged in support in Scotland since a referendum on independence last year, and is predicted to win most of Scotland's seats and become the third-largest bloc in parliament.

Their success has come at the expense of Labour's long dominance in Scotland.

On Monday, the head of the Scottish Labour Party Jim Murphy and party supporter actor-comedian Eddie Izzard, were forced to abandon a street appearance in Glasgow after a confrontation with angry hecklers.

It followed a boost for Miliband when comedian and activist Russell Brand dropped his anti-voting stance and endorsed Labour in a tweet to his 9.6 million followers.

‘I think this bloke will listen to us,’ Brand told his followers in a YouTube video from his bed.