Labour is dropping leaflets accusing the Conservatives of a "secret plot" to work with UKIP in marginal seats it hopes to take from David Cameron's party.
The leaflets, which mirror Tory attack adverts against Ed Miliband and SNP leader Nicola Sturgeon, feature the Prime Minister standing beside Nigel Farage outside No 10.
They ask "Is this what you want to wake up to on Friday morning?"
The leaflets go on to claim: "The Tories are plotting a secret deal with Ukip.
"Only a vote for Labour can stop them."
Labour hope that the leaflets will convince undecided swing voters to back Ed Miliband's party.
Experts warn the party has to win a number of Tory-Labour marginals, especially in London, to have any hope of winning a majority at Westminster.
Senior Conservatives have consistently refused to rule out a deal with Mr Farage's eurosceptics.
Mr Cameron has repeatedly declined to rule out any post-election alliance with the party.
Earlier this year party chairman Grant Shapps did rule out a coalition with Ukip, saying: "I can rule out - We are not going to do pacts and deals with Ukip."
That view appeared to be echoed by the party's chief whip Michael Gove, who said that the Tories would not get "into bed" with Ukip.
But other more senior figures in the party have refused to back that line.
Lib Dem leader Nick Clegg has challenged Mr Cameron to publicly rule out any agreement with Ukip, telling him not to treat voters as if they are "stupid".
The Conservatives have come under fire in this election for their posters depicting various SNP figures 'controlling' Mr Miliband.
One notorious example, also used in marginal constituencies, featured former First Minister Alex Salmond as a pick pocket.
Ukip has said the price of its support would be an early referendum on the UK's membership of the European Union.
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