The Conservative party has released a campaign poster that depicts Ed Miliband in the pocket of the former Scottish National party leader Alex Salmond, sparking a spoof version from Ukip leader Nigel Farage.

The Tory poster, designed by the advertising agency M&C Saatchi, is one of a number of campaign posters that try to scare voters about parties that might form an alliance with Labour in the event of a hung parliament.

Within hours of its release, Ukip released a mock version suggesting David Cameron was in the pocket of European commission chief Jean-Claude Juncker.

Farage himself tweeted an image of a miniature David Cameron peering out of Juncker’s jacket in a move reminiscent of the poster wars between Labour and the Conservatives at the last election.

Nice posters from the Tories today. It would be a great shame if someone spoofed them... pic.twitter.com/C83H1ELAdt — Nigel Farage (@Nigel_Farage) March 9, 2015

Back in 2010, Labour portrayed Cameron as Gene Hunt, the politically incorrect police chief of the Thatcher era, from the BBC’s Ashes to Ashes programme, with a slogan saying “Don’t let him take Britain back to the 1980s”. The Conservatives hit back with a spoof version of the poster, saying “Fire up the Quattro, it’s time for change”.

The new Conservative poster showing Miliband and Salmond will appear on billboards across the country, but all parties are likely to experience their adverts being altered and mocked on social media.

The Tory advert was released after Cameron on Saturday called on Miliband to explicitly rule out any deal with the SNP “if he cares about this country”. The Scottish nationalists are likely to take many of Labour’s 41 seats in Scotland in May.

“If you thought the worst outcome in this election is a Labour government led by Ed Miliband, think again,” Cameron said. “You could end up with a Labour government led by Ed Miliband, propped up by Alex Salmond and the Scottish National party. You could end up with an alliance between the people who want to bankrupt Britain and the people who want to break up Britain.”

NIGHTMARE! Ed Miliband as PM, propped up by Alex Salmond ‘calling the tune’ if Labour gets into Downing Street pic.twitter.com/uNFHs5deTo — Grant Shapps MP (@grantshapps) March 9, 2015

Shadow chancellor Ed Balls said coalition with the SNP was “not part of Labour’s plans”.

Asked if he could categorically rule out a deal with the nationalists, Balls said: “The SNP have said they don’t want a coalition. It’s not part of our plans. We don’t want one, we don’t need one, we’re not after one.

“No large party in the last 100 years – Labour or Conservative – has ever fought a general election on the basis they wanted a coalition or deal with a small party.

It’s the last thing we want. What we want is a majority Labour government.”

Labour has previously argued that the focus on a possible SNP deal is a tactic devised by the Tory election strategist, Lynton Crosby, to deflect attention from issues such as the NHS and the economy.

Labour has repeatedly refused to comment on any possible coalition deals, insisting it is aiming for a majority. The SNP has ruled out a coalition with the Tories and has said it would be unlikely to back a formal coalition with Labour, but it may support the party on an issue-by-issue basis.