Donnie MacLeod confessed to damaging the crops when he was called to give evidence at the trial of another man.

However, he refused to tell Sheriff James Fraser who else had been involved.

He was told to consider his position over the weekend - but again declined to incriminate anyone else when he appeared at Dingwall Sheriff Court on Monday.

The 53-year-old organic farmer from Ardersier, who is chairman of the Highlands and Islands Organic Association, was then held in contempt of court and jailed for 21 days.

His supporters have now promised to stage a vigil outside Porterfield Prison in Inverness until MacLeod is released.

The farmer had been giving evidence at the trial of 31-year-old Matthew Herbert, from Oxford.

He denied damaging crops at Jamie Grant's Rhives Farm, near Munlochy, on 7 June last year by trampling an X-shaped design on the field.

In an email to the media, the GM crop protesters said the "X" was the first vote cast in the General Election.

Mr Grant, a tenant farmer, told the trial on Friday that the mark had been made on the wrong crops.

MacLeod, who had been quoted in the protesters' email, then took the stand and confessed that he had been the culprit.

But he refused to say whether anyone else in the court had helped him, saying: "I don't want to incriminate anyone."

He again refused to answer the question when he appeared in court again on Monday.

"I have thought a lot about it and I have to do what I think is right," he said.

"I am on oath and I take that seriously but I have difficulty in criminalising people.

"I wish no disrespect to the court. I can assure you that I do not offer myself for martyrdom."

Sheriff Fraser said: "This is a blatant and direct contempt of court.

'Moral stance'

"A legitimate question has been put to you which you have refused to answer. A court cannot operate in that situation."

The case against Mr Herbert was dropped. Speaking outside the court, he said: "I find the whole thing farcical.

"Donnie has gone to jail for taking a moral stance in trying to do what is right."

MacLeod is understood to be considering an appeal.