Brian Wright, a former teacher at Park Mains High School where Mr Alexander was a pupil, reported Cllr John Caldwell to the police when the Labour man was out campaigning on behalf of the party’s candidate for Paisley and Renfrewshire South.

Mr Wright, 59, claims Mr Caldwell, a Johnstone councillor, left a “huge puddle of urine” in his bin shelter which sits just yards away from his front door in Elderslie.

But he has yet to receive an apology and pressure is mounting on Cllr Caldwell and Mr Alexander to say sorry.

Cllr Andy Doig, a Johnstone councillor for the SNP, said: “The whole situation is quite sad.

“I understand it can be difficult for men who are middle-aged and may suffer from health problems however I’m bemused at the fact he didn’t just knock on a constituent’s door and ask if he could use their facilities.

“I have a degree of sympathy for Mr Caldwell but I think he has made a tremendous lapse of judgement.

“But as far as I’m concerned he should at least offer an explanation or apologise to the resident.” However SNP member Kenny MacLaren went further, calling on the depute provost to step down.

He said: “It’s hard enough to believe that an experienced man like John Caldwell has done this in the first place without even having the decency to say sorry to the neighbours afterwards, which is inexcusable.

“He’s not a teenage boy and even if he was, it would be shocking enough. This is a mature man, a depute provost, and he really should have known better.

“The first thing he must do is say sorry and then I do believe that he should seriously consider his position as depute provost.

“I think Douglas Alexander also has a duty to act. It could absolutely ruin his campaign.” Mr Wright was at home in Glenmalloch Place when he noticed Labour campaigners entering his street in the Paisley and Renfrewshire South constituency where Mr Alexander has been MP since 1997.

Mr Wright says they were going from door-to-door handing out leaflets and talking to residents for around 10 to 15 minutes.

He claims he spotted Mr Caldwell emerging from the rubbish area with a “satisfied look on his face”.

The former teacher, who taught in Renfrewshire schools for 25 years, went outside to challenge the activist but claims he was dismissed by MSP Hugh Henry who was also part of the group.

Brian, 59, was so disgusted by the alleged incident that he reported the deputy provost to police and Renfrewshire Council.

He said: “I was having a cup of tea and reading a book when I saw a man walking into the bin shelter.

“I didn’t think it was suspicious at the time and just assumed he was going to put some rubbish in the bin.

“But he seemed to be in there for rather a long time and after a while I began to wonder.

”I could see him shuffling around inside then he walked out with a satisfied look on his face.

“I ran outside and looked in the bin shelter where there was a huge puddle.” The urine bust-up happened on Friday, April 17, as Labour activists were campaigning for Mr Alexander.

Pollsters are predicting the shadow foreign secretary could lose the seat to the SNP’s 20-year-old candidate Mhairi Black.

Brian claims Mr Caldwell, 65 - who lives in the next street up from him - admitted urinating and said: “I shouldn’t have done it.” But he says he was shocked at the response of Hugh Henry, who is Scotland’s shadow justice minister.

He added: “As I approached, I called to Mr Henry and asked if he was in charge and if he could explain why one of his group would urinate in my bin shelter and not knock my door and ask to use the toilet.

“I was forceful but absolutely civil in my tone. His response was not a request for me to clarify, nor an attempt to ask to whom I was referring.

“He chose simply to be aggressive back to an obviously angry man, standing directly towards me and shouting out in return ‘Why are you speaking to me? Why would I know?’ “He came right up and virtually eye-balled me as he continued to shout over me. I should’ve phoned the police there and then, I was furious. I still am at the thought of it.

“Even after the guy admitted it I didn’t get an apology.

“That’s the thing I found most galling and to be honest if any of them had apologised I probably wouldn’t have taken it any further.” Brian - who says he has no affiliation to any party - has also given his version of events to Renfrewshire Council as well as the Commission for Ethical Standards in Public Life and urged them to investigate.

In a letter, he wrote: “On returning to my home following the confrontation on the street, I immediately bleached and cleared the mess then phoned Labour’s campaign office in Paisley to complain.

“I spoke to the MP’s election agent who skirted around ways of trying to say he ‘understood how I was feeling’. Yet, still not the courtesy of an apology on behalf of the Constituency MP for the behaviour of individuals working on his behalf, individuals who lacked the decency to apologise themselves.

“Decency, right enough, is hardly an appropriate word in the context of what happened.” Police Scotland said: “We are aware of a complaint and enquiries are continuing.” We invited Cllr Caldwell and Douglas Alexander to comment however both failed to respond before our deadline. Hugh Henry chose not to comment because there is an ongoing police investigation.