The elections watchdog will not be launching a full investigation into “malicious” claims the SNP illegally colluded with a pro-independence business group before last year's referendum.

The Electoral Commission said there was no evidence to support the claims.

Peter Murrell, Nicola Sturgeon's husband and the party's chief executive, was reported to the watchdog over allegations that he had helped run the Business For Scotland (BFS).

The practice would been illegal under the 2013 Scottish Independence Referendum Act (SIRA). The SNP and BFS dismissed the claims as nonsense.

The Commission said it had “found no evidence during its assessment that the SNP and BFS worked together in a way that broke the law. There is therefore no need to open a full investigation”.

It explained: “Under SIRA, if campaigners worked together under an ‘agreed plan’, spending related to this plan would count towards each campaigner’s spending limits. The Commission found no evidence that BFS and the SNP were operating under such a plan.”

The watchdog added: “Should further information come to light, the Commission will consider this but at this stage no further action will be taken.”

The claim is thought to related to a leaked email, which emerged from the controversy surrounding the property dealings of former Nationalist MP Michelle Thomson, who led BFS at the time of last year's referendum.

She has denied any wrongdoing and withdrew from the SNP whip when a police investigation was launched.

A BFS spokesman said: “These were malicious and sensationalist claims made by the Unionist press hoping to damage the Yes movement and we welcome the Commission’s confirmation that we acted entirely properly.”

Stressing how the group was never in any doubt it had acted within the law, he added: “They told us that speaking with, sharing a platform with and getting updates from other Yes groups was, not only within the rules, they expected us to do so."

An SNP spokesman said: “This confirms what we have said throughout, as the Electoral Commission guidance makes clear, informal discussions with other campaigners are not covered by the provisions."