POLICE have been asked to investigate a smear campaign linking the pro-Union Better Together to far-right parties.

A four-page letter purporting to come from Better Together sent out to politicians and known supporters of the Union put the logos of the National Front and British National Party on the letterhead alongside the mainstream parties who actually comprise the campaign, Labour, the Liberal Democrats and the Conservatives.

UKIP and the Scottish Unionist Party logos were also used on the letter which becomes a satirical attack on the pro-Union case which one recipient said he found offensive and threatening. The envelope in this case was posted through a specific pre-paid business account, which The Herald has established belongs to a major UK firm.

The hoax mixes known facts, smears and obviously humorous material. It gives as its address the United and Cecil Club in Sussex, a genuine Tory fund-raising operation.

The letter opens: "Dear fellow Unionist, Your name has been forwarded to us as a true British patriot" and speaks of the coming "Battle for Britain". But it then repeatedly uses the word Scotch to describe Scots interests, attacks Alistair Darling over his Westminster expenses and political record, smears former LibDem leader Charles Kennedy and criticises Ruth Davidson for her private life and support for same-sex marriage. It is signed Better Together (The Unionist Campaign).

Doug Maughan, a critic of independence, said: "I assume I received this because I've had letters published in The Herald, arguing that Scotland's future is best served by remaining as part of the UK.

"Someone's obviously put a significant amount of time and effort into producing the letter, and I wonder what else they might be capable of. I'm sufficiently alarmed that I've discussed it with the police and will be making a formal complaint with them."

Better Together said: "This ridiculous fake letter, coupled with a co-ordinated effort by an extreme SNP group to disrupt public events is an affront to democracy."

The SNP said: "This is to be deplored, and it's obviously got nothing to do with the No or the Yes campaign."

Yes Scotland responded: "We have no knowledge of this. Yes Scotland will always run a clean and honest campaign."