The hot-headed 25-year-old admitted assaulting former Manchester City team-mate Ousmane Dabo at a court appearance on Monday.

He is still to be sentenced for the "snarling attack", after appearing before a judge in Manchester's Minshill Crown Court.

Barton changed his plea to guilty at the last moment after Judge Khokhar Mushtaq indicated that he would receive a suspended sentence if he did so.

Prosecutor Richard Varden said Barton punched Mr Dabo, 31, a French international, after confronting him during a training session.

The blow to the temple caused Mr Dabo to fall backwards, only for enraged Barton to hit him a further four of five times.

He said there had been a "degree of unpleasantness" between the two, and that a number of heavy challenges had been made by both men.

Barton, who has admitted having a drink problem, was produced from Walton prison on Merseyside, where he is serving six months for a booze-fuelled attack on two people carried out in December.

That offence was committed while he was on bail for the assault on Mr Dabo.

He is eligible for early release, subject to him wearing an electronic tag.

But professionally, it could be the end of the line for the £60,000-a-week player, who now faces having his contract ripped up by furious club owner Mike Ashley.

Even if he stays, Barton - who refused to take a 50 per cent pay cut during his time inside - will have to endure the taunts and hostility of Toon fans, many of whom were sickened by the attack.

The club has not yet commented on Barton's latest conviction, but is expected to issue a statement once sentence has been passed on the Dabo incident.

The club will look to sit down with Barton and his representatives as soon as possible to decide on the best way forward.

Barton was Sam Allardyce's No 1 transfer target last summer, and, as yet, Newcastle fans have seen just glimpses of why he spent £5.8m – including a £300,00 "loyalty" payment Man City were withholding from him – to bring him to St James's Park.

And Ashley effectively now has to decide whether to write off that sizeable investment, made in the last days of his predecessor Freddy Shepherd's tenure, by terminating Barton's five-year contract.

Such a course of action, however, would give manager Kevin Keegan – who pledged to stand by Barton, who is committed to receiving help from Tony Adams's Sporting Chance clinic in Hampshire, last season – yet another unwanted headache.

Keegan – still to sign a player this summer – has already lost four players since the end of the season, and he will be reluctant to lose another given his limited transfer budget.