Saturday's 3-1 home defeat to Bournemouth saw anti-McClaren chants and boos from Magpies supporters ring around St James' Park.
It was the Tyneside club's 16th Barclays Premier League loss of the season, their fifth in their last six games, leaving them second-bottom of the table with 10 matches to play.
McClaren vowed in his post-match press conference on Saturday that he would not be resigning, but talk is rife that the 54-year-old is facing the sack, with former Newcastle owner Sir John Hall on Sunday saying he should be axed.
While Newcastle are yet to comment on McClaren's position, reports on Monday suggested the board, of which he is a member, was holding discussions about the situation without him.
Meanwhile, an open letter to the club from the Fans Forum - which has 15 seats occupied by supporters representing different elements of Newcastle's fanbase, and meets to put questions to club officials at various points during a season - was published.
It said: "What NUFC fans want is a team of 11 players in black and white shirts that understand their role to perform to their maximum - week in, week out.
"We have entrusted the board of NUFC to ensure that the right personnel are in place to carry out this basic expectation.
"Whilst we still have 10 games to go, NUFC Fans cannot accept the current work ethic of the players.
"Someone, somewhere has to be accountable and decisions must be made.
"We must urge you to review our position and initiate any changes you see fit to correct the current attitude of the playing staff.
"Our players frequently look jaded and our playing style often appears outdated. We lack organisation and the players continue to lack discipline.
"Maybe it's time to accept that the 'Head Coach' role simply doesn't work at NUFC and accept that a Manager is required, a person who will not only coach but motivate and manage!
"We want players that are proud to wear the famous colours. We want a manager that truly wants the best for us.
"It's time for action, it's time for accountability."
The letter also listed a series of negative statistics Newcastle have accumulated this season under McClaren, who was appointed as boss last summer after being sacked by Derby.
Included were the facts the Magpies have won only six league games from 28 so far to match their total of victories at the same stage in their relegation campaign of 2008-09, that they have the second-worst goal difference in the division, and that they have only managed to score 28 goals.
The emergence of the letter came after further fierce criticism of Newcastle from their former player Alan Shearer, who had a brief stint managing the team for the final few games of the 2008-09 season.
Shearer told BBC's Match of the Day programme on Saturday: "The whole club is a mess from top to bottom."
And the 45-year-old followed that up in his column for The Sun, saying: "The most shameful slur anyone can possibly aim at a professional player is that they simply don't care about their club.
"But today that can be aimed at 90 per cent of Newcastle's players.
"Their pathetic performance against Bournemouth proved it once and for all."
Shearer suggested league leaders Leicester "could rack up a cricket score" against Newcastle next Monday, predicted the Magpies would lose the derby clash with Sunderland after that, and said he was "convinced" they were heading for relegation.
"Quite simply they (the players) are not playing for McClaren or the Newcastle fans," Shearer said.
"Steve was very lucky to get this job after overseeing Derby's implosion last season.
"For some time now he has looked powerless to stop a slide into the Championship. Because that is where I am convinced my club is heading now.
"And don't believe that the size of the club means they will be bouncing back any time soon either.
"They could go right through the Championship as well, because there are barely any characters in this side who would fight to get the club back up. They simply do not care.
"They have betrayed a club, a city, a community, with their lacklustre and totally heartless performances.
"They may even be responsible for losing a generation of fans."