Lawyers acting for SNP Westminster leader Ian Blackford have demanded an apology and threatened to sue a Tory MP who described him as an arms dealer.

Solicitors acting on behalf of the SNP MP  have written to Ross Thomson after he tweeted a jibe at Blackford’s expense on Sunday.

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Thomson tweeted a picture of the article with the line: “Just a simple arms dealer,” alongside a shocked-face emoji following reports that the investment fund Ian Blackford chairs profited from defence deals.

Ross Thomson deleted the tweet two hours later.

According to reports in the Mail on Sunday, Golden Charter Trust indirectly benefits from arms manufacturers including BAE Systems, Raytheon, Airbus, Northrop Grumman and Harris Corporation, all of which have ties to Saudi defence.

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According to the Times, solicitors acting on behalf of Blackford have written to Thomson.

A senior SNP source told the paper “His false and defamatory statement will require an apology and a charitable donation. Ross has received the letter and will respond in due course.”

Blackford’s register of interests states that he earns £3091 a month for 32 hours’ work a year as the trust’s chairman.