Voters could be forced back to the polls before Christmas

The Liberal Democrat leader warned that only a coalition involving his party could provide 'stable' government after the May 7 vote.

Any attempt by the Conservatives or Labour to run a minority administration with the support of smaller parties such as UKIP or the Scottish National Party would be 'a shambles' and would force voters back to the polls before the end of the year, he predicted.

The deputy prime minister's words came as David Cameron appeared to indicate that he was ready to seek a new coalition in the case of a hung parliament, saying he would 'always put the country first'.

Meanwhile, shadow health secretary Andy Burnham said Labour would do 'whatever we can' to put its agenda before Parliament in a Queen's Speech, prompting Tory claims that Ed Miliband's party would attempt to form a government even if it meant being propped up by the Scottish National Party.

In an apparent signal that he was open to a fresh coalition with Tories, Mr Clegg said that avoiding an in/out referendum on Britain's EU membership would not be a 'red line' deal-breaker for Liberal Democrats in any coalition deals.

While both of the biggest parties insisted they were fighting for an absolute majority, the latest slew of polls continued to point towards a neck-and-neck finish and a hung parliament.