Labour's Holyrood leader Wendy Alexander was accused yesterday of going on a month-long "self-imposed silence" since the Electoral Commission began investigating an illegal donation to her leadership campaign.

The SNP's Roseanna Cunningham said that by Friday a full month would have passed without the Labour leader giving any interviews or answering any questions.

But Labour sources accused the Perth MSP of showing a lack of respect for due process now that the Electoral Commission is investigating the matter.

Ms Cunningham said: "It is ridiculous that the leader of Scotland's opposition has maintained her vow of silence for nearly a month. What kind of leadership or opposition is that? Silence is golden but Scotland deserves better from the leader of the opposition."

The Herald broke the story on November 29 about the donation of just under £1000 from Jersey-based businessman Paul Green to Ms Alexander's leadership campaign coffers.

As he cannot register to vote in UK elections he is barred from making political donations, but Ms Alexander's campaign team went ahead and solicited the money.

Cathcart MSP Charlie Gordon paid for that with his place in Labour's shadow cabinet. Ms Alexander insisted that she had e-mails which cleared her of any involvement in the illegal donation, although she sent a handwritten note to Jersey thanking Mr Green.

Ms Cunningham said yesterday: "Wendy's problems have made her a lame duck leader and it's time both she and the Labour Party thought long and hard about this strategy.

"With Scottish MPs rebelling against her leadership, Wendy Alexander seems increasingly isolated.

"The Electoral Commission must report as early as possible and as Ms Alexander and her team have admitted breaking the law it seems the only place for this investigation to go is either straight to the police or the procurator-fiscal. Breaking the law is breaking the law."

But a Scottish Labour spokesman insisted last night: "Wendy Alexander has fully co-operated with the Electoral Commission. Due process is under way and must be respected."

The Electoral Commission has been unable to give any estimate of how long its investigation will take, or whether it is likely to be followed by a referral to the police. The issue of an illegal donation to Ms Alexander's leadership campaign could not have come at a worse time for Labour, which was coming under fire for accepting £600,000 in donations through proxies acting on behalf of the North of England property developer David Abrahams.

The Metropolitan Police are investigating whether anyone should be charged over this illegal use of go-betweens.