Magpies officials reacted quickly yesterday morning after Barton posted a series of critical comments on his Twitter site on Sunday night.
The 28-year-old midfielder is also understood to have launched into a post-match tirade in the Elland Road dressing room after Sunday's 3-2 defeat to Leeds, an incident that stiffened Newcastle's resolve to move him on.
Barton, who entered the final year of his current contract last month, has been told to seek alternative employers and promised a free transfer away from Tyneside.
The decision, which was confirmed via an official statement later in the afternoon, has alerted a number of top-flight clubs, who are prepared to offer Barton an opportunity to rebuild his career despite his chequered past.
Arsenal, Everton and Manchester United have all been linked with the England international already this summer, but it is understood Aston Villa and Tottenham are the early leaders in a chase that is likely to be resolved before the weekend.
Spurs chairman Daniel Levy is understood to be the first person to have inquired about Barton's position yesterday.
Villa boss Alex McLeish has courted Barton in the past, and the Midlands club will be able to match the midfielder's wage demands, which are likely to be more than £40,000-a-week, following the recent sale of Stewart Downing.
Newcastle would like to remove Barton's wages from their own books as quickly as possible, as that would enable Alan Pardew to recruit a replacement before the start of the new Premier League season.
Switzerland international Tranquillo Barnetta is the primary target, as members of the Magpies' scouting team watched him on a number of occasions last season.
Barnetta, who scored both of Switzerland's goals in June's 2-2 draw with England, would be a like-for-like replacement on Newcastle's right flank.
Barton's final appearance in that position came in Sunday's friendly defeat, a match that precipitated the Twitter outburst that ultimately proved the final straw in terms of the club's tolerance of his increasingly unpredictable behaviour.
The midfielder said: If only we as players could tell the fans exactly how it is, without them above fining us lots of money.
The comments reflected a mounting frustration at the Newcastle hierarchy's reluctance to invest in their playing resources, and came in the middle of a pre-season period that, even by the Magpies' standards, has been almost comically chaotic.
From players, including Barton, being unable to travel to the United States to a game being changed to a plastic pitch at the last minute, Pardew has encountered a series of obstacles as he attempts to prepare his players for the new campaign.
Throw in a long-running, and so far futile, attempt to recruit a new striker, the impending departure of Jose Enrique and a less than convincing explanation of where the £35m received from the sale of Andy Carroll has gone, and you have a scenario where the mood amongst supporters is as acrimonious as it has been at any stage of owner Mike Ashley's unpopular reign.
Barton's own unhappiness was evident last January, when talks over a new contract broke down on the same day that Carroll was controversially sold to Liverpool.
Barton felt Newcastle had reneged on their previous promises, and with the two parties adopting an increasingly antagonistic stance, attempts at renewing discussions in the second half of last season failed.
There were no attempts at a reconciliation this summer, indeed Barton's agent, Willie McKay, was summoned to St James' in May to be told there would be no new contract offer.
Barton was furious at the move, and expressed further frustration when his dressing room confidante, Kevin Nolan, was sold to West Ham United.
His discontent, which initially remained private, burst into the open over the weekend, and Newcastle officials clearly decided that the best way to avoid an outright civil war was to wash their hands of the one-time England international.
On the one hand, their move was understandable, as Pardew clearly had no control over Barton's actions and, judging by the reaction of some of his team-mates on Twitter, other players were ready to rally to the Liverpudlian's cause.
However, the granting of a free transfer threatens to rip Newcastle's dressing room apart and robs the club of last season's most influential player just 12 days before the opening game of the new campaign. Tellingly, Newcastle did not win a single Premier League game that Barton was not involved in last season.
Alarmingly, yesterday's developments also appear to undermine Pardew's position, as the manager has repeatedly expressed a desire to keep one of his club's most important players.
Little more than a week ago, Pardew said: I think Joey will be here (next season). He's said all along he wants to stay and I want him to stay so there's no conflict.
Following confirmation of yesterday's decision, Barton's online reaction was somewhat muted, although further parting shots are anticipated once a move away from the North-East is secured.
Barton issued three Tweets that said: Somewhere in those high echelons of NUFC, they have decided I am persona non grata.
I am on a free but the honour of wearing those B+W stripes surpasses that.
One day the board might realise what the shirt signifies. HONOUR and PRIDE. Thanks for your continued support. #toonarmy
McKay, meanwhile, branded Newcastle's actions "suicidal", and predicted he would be able to find his client a leading Premier League club.
"I think it's a wee bit suicidal by Newcastle because how would you let your Player of the Year leave on a free transfer, approaching his prime," he said. "I just can't understand what Newcastle are doing.
"Derek Llambias and Mike Ashley stood by Joey when he was under real pressure so I'm not going to come out and slag off Newcastle's board but I think they have made a mistake.
"He was going to take a drop in wages to sign a new contract."