The prime minister has been warning for weeks about the danger of a Labour-SNP alliance in a series of attack ads showing his opponent Ed Miliband in the pockets of Sturgeon and her predecessor, Alex Salmond.
The London mayor, Boris Johnson, made the attacks even more personal on Monday, describing Sturgeon as a “scorpion” and likening her to King Herod – who, according to the Bible, had all the baby boys in Bethlehem slaughtered following the birth of Jesus.
Cameron, who was speaking in south Cheshire, said: “Make no mistake, if Labour and the SNP get into power, you are going to see an alliance between a party that wants to spend, borrow and tax more with a party that wants to spend, borrow and tax even more.
“It might be a match made in heaven for them, but it is a match made in hell for the British economy.”
He said such an alliance would lead to “endless bargaining and backroom deals. Every single decision taking the form of a ransom note – written by the SNP”.
Meanwhile, Johnson launched his first attack in the Telegraph, writing: “You wouldn’t get Herod to run a baby farm, would you? It would not normally occur to you to interview a convicted jewel thief for the post of custodian of the Tower of London.
“Any such course of action would be totally nuts. So can someone tell me why in the name of all that is holy there are some apparently rational people who are even contemplating the elevation of the Scottish National party to a position of effective dominance in the government of the United Kingdom – an entity that they are sworn to destroy?”
Later, speaking to the Guardian next to a replica Spitfire outside the RAF bunker where Winston Churchill oversaw the Battle of Britain in 1940, he also compared Sturgeon to “a scorpion”.
He was responding to a question about reports that the SNP would block defence spending under a Labour government unless it wins over the replacement of the Trident nuclear deterrent.
“People need to wake up to the reality that there is no way to have a Labour government except as the puppet of Scottish nationalists,” he said. “There’s nothing wrong with the Scots having a strong effect on British life, of course they should. The issue you have is you have a party whose aim, ethic, charter and mission is to destroy the United Kingdom.
“They have no interest whatsoever in the political health of Ed Miliband or the UK. People need to realise that Miliband is in the position of the frog carrying the scorpion on his back. You can see already the ransom they are starting to demand on issues like defence and welfare, reforms which the vast majority of moderate Labour supporters believe in.”
Sturgeon has dismissed the attacks, saying Johnson’s words in particular were an “entirely offensive comment”.
At the SNP’s manifesto launch she said: “I think it will be treated as that not just by people in Scotland but across the UK … In my experience, ordinary people the length and breadth of the UK do not see Scotland that way at all and do not see the SNP in that way at all.”
She said the proposal announced by Cameron on Monday for an annual audit of the impact of Scottish policies on the rest of the UK would appal people in Scotland and was born out of panic and desperation.
The Ukip leader, Nigel Farage, also attacked the SNP on Monday. He said: “David Cameron, Ed Miliband and Nick Clegg have all shown appalling weakness in the face of the aggressive tactics of the SNP which threaten the future of the United Kingdom.
“All three of them have guaranteed the continuation of the Barnett formula in its present guise, ripping off English voters and throwing truckloads of English money over Hadrian’s wall. All three signed that appalling pledge just before the referendum, promising the Scots almost anything they wanted. Well, I didn’t and I intend to speak up for England.”
Taking a similar line to Cameron, he made a particular appeal to Labour voters to switch to Ukip to “stop the nightmare of a Labour administration that depends on the SNP for its survival”.
“Everyone knows that it will be Nicola Sturgeon who wears the trousers in that relationship,” he added.
This article was written by Rowena Mason and Robert Booth, for theguardian.com on Monday 20th April 2015 15.33 Europe/Londonguardian.co.uk © Guardian News and Media Limited 2010