The 49-year-old was speaking after what he described as "death threats" towards fellow referee John Beaton following the derby between Celtic and Rangers.

Beaton was given a police escort to Ayr's game against Falkirk on Saturday.

"The volume of criticism and the tenor of some of the criticism is different this year," McKendrick said.

"Most concerningly, there has been physical violence received by referees - the coin-throwing incident - and now the threats of physical violence and death threats to one of our officials."

Assistant referee Calum Spence was hit by a coin during a match between Livingston and Rangers in September.

And Police Scotland is investigating Beaton's allegations that he received threatening texts and phone calls after the referee was criticised for not punishing Rangers' Alfredo Morelos for three incidents during the striker's win over their city rivals last month.

"It shouldn't be happening in Scottish society full stop and, when it gets to the stage where someone is receiving phone calls to their house, it is impacting on their personal life; we have to reflect on that," McKendrick told BBC Scotland.

"It's happened before where referees have received threatening phone calls to their house, but it's sad that it's come back round again."

'When you're threatening referees a line has been crossed'

McKendrick called for "productive steps" to tackle the problem and believes some are already being taken.

The Scottish FA will host a meeting between referees and club representatives this month after a season in which managers have criticised not only individual decisions but general standards of officiating and the SFA's disciplinary process.

"It has been described by some as a summit, but summit is a grand word for it and perhaps over-eggs it a little bit," McKendrick said.

"It is a coming together of people interested in football to think about what we have to do to make it better.

"It's good that we want to come together to look at the productive steps we can make to make football better in Scotland. Maybe it's VAR, or maybe it's other matters to do with how disciplinary matters are handled."

McKendrick insists that referees do not expect to be immune from criticism.

"Referees have no problem whatsoever with people criticising a decision - that's part and parcel of the game," he said.

"It's gone beyond criticising match decisions to criticising competence and the net effect of that is not good for anybody.

"When it gets to the stage where you're threatening referees then that's a line that's been crossed."