LIB-DEM leader Nick Clegg made clear that electoral reform would be an “absolute pre-condition” as he spelt out some of the conditions under which he would offer his party’s support in a hung Parliament.

And he indicated that he would not be willing to prop up Gordon Brown in power if Labour came third in the popular vote but won the most seats.

The Conservatives accused Mr Clegg of trying to “hold the country to ransom”.

And there was doubt over whether the Conservatives would contemplate any post-election pact which involved ditching first-past-the post for Westminster elections.

David Cameron said he wanted to keep the existing system, but avoided explicitly ruling out voting reform.

But he made it more difficult for himself to strike an eventual deal with the Lib-Dems by saying that proportional representation would be “a big, big mistake for this country” and describing their policies as “away with the fairies”.

And the Tories launched an election broadcast warning that a hung Parliament would lead to back-room deals, indecision, weak government and a paralysed economy.