The Scottish Premier League have come under fire from both sides of the Old Firm after announcing plans to extend their season by four days if Rangers reach the UEFA Cup final.
Rangers were last night irate that the season would not be extended further, while Celtic were upset that the fixture schedule could be altered on an 'ad hoc basis'.
Celtic chief executive Peter Lawwell claimed the SPL had 'compromised the integrity of the competition' by agreeing to stretch the season out to May 22.
The change will only be implemented if Rangers defeat Fiorentina in their UEFA Cup semi-final, but the Ibrox club's chief executive Martin Bain was adamant the season should have been extended in any case, as regardless of the result against the Italians, Walter Smith's side face a hectic run of games.
If they beat Fiorentina, Rangers must play three games in five days at the end of their season, culminating in their May 24 date with Queen of the South in the Scottish Cup final.
They already have a backlog and the SPL have been faced with the problem of fitting games against Motherwell and St Mirren into a crammed schedule.
Should they lose to Fiorentina, Rangers must play their last four SPL games in nine days.
Bain accused SPL chiefs of 'doing nothing' to help the club in their UEFA Cup bid, and insisted the SPL season should be extended to May 27.
'We as a club are really bitterly disappointed in the SPL on all fronts,' Bain said.
'It is hardly the best preparation in the lead-up to the UEFA Cup final - if we get there - to play Sunday, Wednesday, Saturday in advance of that.
'It's all very well for the SPL to go on about Rangers' co-efficient and how it's great for Scottish football but they are doing nothing to assist us.'
Rangers face Fiorentina at Ibrox tomorrow in the semi-final first leg, with the Wednesday, May 14 final in Manchester their target.
When Celtic reached the 2003 UEFA Cup final, a fixture reshuffle meant they were afforded a free week before the final.
The SPL unveiled two different fixture schedules. One will take effect if Rangers beat Fiorentina, the other if they fail against the Italians.
Most teams will be affected if Rangers progress, with the final round of top-six games shunted back four days to May 22.
Celtic chief executive Lawwell said: 'We do not want to disadvantage Rangers should they reach the UEFA Cup final, but we are extremely disappointed and concerned at the manner of the decision-making process - which was done without consultation with the clubs or the SPL board - and the effects on the integrity of the competition.'
He added: 'In principle we accept fixture flexibility. However, the league must not be compromised by changing the rules during the course of the competition and these decisions should not be made on an ad hoc basis.
'Therefore, we oppose the decision and feel, in fact, that it has compromised the integrity of the competition.'
SPL chairman Lex Gold said: 'We have tried to be fair to the integrity of the competition, fair to our clubs and fair to the fans.'