Days after Willie Collum received threatening phone calls after handing Rangers a dubious penalty on his Old Firm debut, Clark has warned that Scotland is at risk of a referees shortage.

Assistant referee Steven Craven's resignation from the professional game was confirmed 24 hours after the Parkhead controversy, as a debate over the penalty U-turn in Celtic's previous game turned into a Scottish Football Association investigation.

Craven and his teenage sons were reportedly the subject of threats and verbal abuse after Dougie McDonald rescinded a penalty award to Celtic during their 2-1 Clydesdale Bank Premier League win against Dundee United.

And Clark has warned that other match officials are ready to follow suit.

"I'm aware of several people involved at the top level who are contemplating their futures simply because of some of the abuse that is going about," the former FIFA referee said.

"It's got to be a big concern for everyone in football, not just those involved in refereeing.

"The pressure brought to bear on match officials has intensified enormously, even since I retired two years ago.

"People need to take a step back from that and recognise match officials are an integral part of the game and, if we keep on putting them under so much pressure, we'll end up with a situation where people simply don't want to be involved in refereeing.

"There are those who are currently involved who might be thinking, 'Is it really worth it to continue?', particularly where their families are drawn into the vicious circle of abuse.

"Secondly, it's a problem for referee recruitment. If any young man was to say to his family this week, 'I'm thinking of taking up refereeing', I`m sure his family would try to dissuade him of that notion. And that can't be healthy for the game."

Clark admits he asked himself whether his career was worth the added pressure on more than one occasion, but he feels his successors have been handed an even more thankless task.

"There were a couple of times where I certainly took stock but I was fortunate in that it never impinged to any great extent on my family life," he said.

"I think that's the most worrying aspect. Issues are now being brought to referees' doors, quite literally, and their families are becoming involved and that must make life very, very difficult for the current crop of officials."

McDonald's own future has been called into question with Craven believed to be unhappy to have been portrayed as the instigator of the Tannadice penalty U-turn.

The SFA's referee committee will rule on the incident tomorrow after new SFA chief executive Stewart Regan handed over his investigation to them.

Lawyer Clark, who today helped the SFA make the draw for the third round of the Scottish Cup, did not want to undermine their inquiry but he added: "Dougie is scheduled to be fourth official at an SPL game at the weekend and I fully expect him to be there."

The inquiry began after Celtic wrote to the SFA seeking clarification over the decision.

Manager Neil Lennon also confirmed they had done the same over the Old Firm penalty, when Kirk Broadfoot fell theatrically after minimal contact with Daniel Majstorovic.

Clark called for managers to temper their language about match officials, especially before games, and criticised Celtic for revealing that they had written to the SFA.

Clark added: "The difficulty is clubs further down the line might say to themselves, 'If we had submitted a letter calling into question a referee's decision, would it have resulted in an inquiry?'.

"I think we all know the answer to that."