ERC chief executive Derek McGrath expects Quins to solve the problem by playing one of their three fit props, Jason Leonard, Jon Dawson or Bruce Starr, at hooker.
The club's inability to pick a reserve prop would leave the referee no alternative but to order un-contested scrums in the event of a front row injury.
'We hope Harlequins will fulfil the rules and play,' said McGrath. 'We cannot change our rules and I hope they do not take any decision which would damage the tournament.'
Unless they back down and complete Sunday's fixture, Quins run the risk of being fined and thrown out of Europe.
Scott Gibbs found out yesterday that he faces a six-week layoff after fracturing his thumb against Neath last weekend - he faces a race against time to be fit for the Lions' tour.