DAVID Cameron's election campaign has been "poisonous" for the Scottish Conservatives, a former party press chief has warned.

Andy Maciver, who worked for former Scots Tory leaders David McLetchie and Annabel Goldie, said the Prime Minister's repeated warnings about the prospect of a Labour government propped up by the SNP "might very well" win him the keys to 10 Downing Street.

But he warned the message undermined his party's efforts counter long-running claims it is anti-Scottish.

Mr McIver said senior Scottish Tories were "livid" at Mr Cameron's approach.

"There is mouth-foaming anger amongst those to whom I have spoken," he wrote in a blog for Conservative Home, an influential online forum for activists.

"If the SNP could create the Tory campaign, according to my erstwhile colleagues, they would have created the one we have seen for the last week.

"It has legitimised the left-wing paranoia which is feeding the army of monkeys required by the SNP's organ grinders."

Mr Cameron's campaign has relied on increasingly stark warnings about the prospect of the SNP wielding influence after May 7.

Posters have depicted Alex Salmond first with Ed Miliband in his pocket and more recently as a pickpocket.

On Sunday, Home Secretary Theresa May questioned the legitimacy of a Labour-SNP alliance and claimed it would amount to the biggest constitutional crisis since the abdication of 1936.

Meanwhile Mr Cameron has emphasised the need for English votes for English laws to counter the growing power of Holyrood and has set out a "Carlisle Principle" to ensure decisions at Holyrood did not impact adversely on the rest of the UK.

Mr Maciver who advocates a federal UK, said the Scots Tories were "increasingly being hindered by their association with London".

He wrote: "Without any personal offence intended to any individuals, it is clear to me that Downing Street has absolutely no strategy whatsoever when it comes to Scotland.

He said the Conservatives argued the UK was a "family" of nations to win last year's referendum but were now raising fears of Scottish influence.

He said the campaign undermined Scots Tory efforts to "fight back against 30 years of SNP assertion that the evil Tories are anti-Scottish."

He wrote: "These Scottish Tories wanted to see their vibrant, exciting leader (Ruth Davidson) talking up the government's job creation miracle, tax cuts and public service reform.

"Instead, they woke up to find that, while they were sleeping, the Saltire they've been waving has been replaced by the St. George's cross."

He added: "The last week might very well help David Cameron back into Downing Street by pulling England towards the Tories and pushing even more of Scotland into the welcoming arms of the SNP, both at the expense of Labour.

"However, it has been poisonous for the Union, and poisonous for the Scottish Conservative Party."

A number of senior Conservatives have warned Mr Cameron's strategy will strengthen the SNP and put the Union under strain.

In a speech last night, Gordon Brown, the former Labour prime minister, accused the Conservatives of turning the election into a battle between England and Scotland, to the Nationalists' advantage.

A Scots Tory source said: "This is a mis-timed blog on the day we rose five points in the polls."