Police have been called in after former Scottish secretary Alistair Carmichael received a death threat.

The letter was sent to the Liberal Democrat's constituency office in Orkney on Thursday.

The threat is believed to relate to Mr Carmichael's backing for air strikes in Syria. Officers have deemed it serious enough to offer him safety advice.

It is the latest blow for the MP, who has faced a public backlash after admitting responsibility for leaking a memo written by a civil servant which wrongly suggested First Minister Nicola Sturgeon wanted David Cameron to win the May election.

Last week he was cleared in a special election court of deliberately misleading voters over the leak, although the two judges concluded he had told a "blatant lie" in a television interview when he denied having prior knowledge of the memo leak.

The action was brought by a group known as the 'Orkney Four', who crowd-funded the legal action.

A Scottish Liberal Democrat spokeswoman confirmed the death threat did not refer to the memo leak.

Police are understood to have ruled out any link to terrorism in connection with the letter.

Mr Carmichael said he could not comment while the police investigation was ongoing.