England's increasingly bitter relationship with the International Board was heading towards meltdown last night after the world governing body imposed a £60,000 fine on the English Rugby Union which in turn accused the IB of holding an improperly constituted hearing and failing to act impartially.

At the end of its two-day meeting in Dublin the IB handed down the fine after declaring Twickenham guilty of allowing the top English clubs to pay unauthorised matches against the Welsh rebels Cardiff and Swansea who have already been fined £150,000 by the Welsh Rugby Union.

England's leading clubs have also fired off an uncompromising warning to the WRU, condemning its "aggressive action" against the Welsh rebels who have already been pencilled in by the English clubs as potential members of the new British League next season. English First Division Rugby, the clubs' umbrella company, described the swingeing fine on Cardiff and Swansea - who have said they will not pay - as "extremely unnecessary and particularly unhelpful" while talks on the new league are going ahead.

Twickenham's escalating dispute with the IB looks almost certain to end up in the High Court after Francis Baron, the RFU chief executive, acting on legal advice, vigorously objected to the constitution and procedure of the IB's five-man disciplinary panel, forcing the withdrawal of one member, Alan Sharp of Canada.

The IB has reserved judgment on another charge that the English RFU was in breach of regulations in permitting a complaint by its clubs to the European Commission on the IB to go forward.

In a bizarre twist, the IB has accepted that Twickenham did not actually authorise the Anglo-Welsh fixtures involving Cardiff and Swansea and also refused to appoint match officials when they went ahead.

Yet the IB disciplinary panel, chaired by Tim Gresson of New Zealand, still decided a £60,000 fine was appropriate when the RFU admitted it "had turned a Nelsonian blind eye to the matches". The money will be withheld from a grant the IB was due to make to Twickenham next month.

Baron, though, promptly put the governing body on the back foot by calling into question the entire legal standing of the Dublin meeting. "We and our legal advisers remain firmly of the belief that the whole process used by the IB in this case is irretrievably flawed," said the RFU chief executive.

He went on: "We are not prepared to let matters stand as they are and we have advised the IB disciplinary committee of this fact. The IB should be setting standards for the game in terms of due process. This was an example of how things should not be done and brings no credit to the game or our governing body."

The RFU backed up its counteroffensive, insisting that it "does not consider that the disciplinary committee has either been properly constituted or validly convened, nor does it believe it to be impartial."