Premier Rugby, an organisation representing the 12 Guinness Premiership clubs, says it will start legal proceedings unless the situation over payment to three clubs is resolved within seven days.
The dispute centres around a £120,000 pot which has been withheld from Sale, Leicester and Wasps. The RFU say the three breached an agreement over the amount of rest players would be given upon their return from the tour. Premier Rugby deny any such deal existed.
A spokesman for the umbrella organisation said: “We do not accept that an authorised agreement was reached on the rest periods for English Lions."
The RFU added: “The RFU confirms that it has received a letter from lawyers acting for Premier Rugby Limited advising that legal proceedings will be initiated against the RFU within seven days, unless certain undertakings are received.
"The RFU has referred the matter to its own lawyers."
