The squad salary caps agreed by League One and Two clubs are "unlawful and unenforceable", according to the Professional Footballers' Association (PFA).

A meeting on Friday saw clubs in the third and fourth tiers back the proposals - although Oxford United were among the clubs to vote against them.

It means in League One, spending on areas such as wages, bonuses - with the exception of promotion bonuses - and fees to agents is capped at £2.5million.

U's boss Karl Robinson was critical of the plans, saying they would create a huge gap between League One and the Championship.

The rules came into force immediately according to the English Football League (EFL), but the PFA have served notice of arbitration and say until that process is complete, the rules cannot take effect.

A statement from the players' union read: "We are disappointed at the outcome of today's votes.

"The EFL has ignored its legal obligation to consult with the PFA and the Professional Football Negotiating and Consultative Committee.

"As such, the legal advice we have received is clear that the salary cap envisaged by the EFL would be unlawful and unenforceable.

"The PFA has already served its notice of arbitration on the EFL and until such time that arbitration is determined one way or another the new regulations should have no effect.

"While we share the league's commitment to protecting the long-term sustainability of the leagues, the salary cap proposals voted on have been rushed through without the proper consideration or consultation."

Players under 21 are exempt from the plans, while anyone on contracts signed before Friday will be treated as being paid at the divisional average - just over £110,000.

Clubs who spend more than the cap will be taxed on a sliding scale, with anyone above five per cent over the limit being referred to a disciplinary commission.

While the PFA opposes the move, the chair of the parliamentary committee which last month recommended greater cost controls across the EFL has welcomed the adoption of the rules as "common sense" and urged Championship clubs to follow suit.

It is understood second-tier clubs have no plans at this stage to hold a formal vote on a cap, but Julian Knight, the chair of the Digital, Culture, Media and Sport committee says they have to act.

He told the PA news agency: "It seems there is a breaking out of common sense amongst league clubs.

"For a long time the flawed business model of much of football has been as clear as day but it's taken the biggest financial crisis in English football since the war for some action.

"Let's see if more realism now permeates the Championship, where player wages incredibly outstrip club turnover."