Saracens are in advanced talks to sign South Africa lock Eben Etzebeth on a short-term deal.
Etzebeth is set to move to England following his international matches in November.
The 24-year-old is contracted to the Stormers and the South African Rugby Union, but last year had a three-month spell in Japan during the southern hemisphere off-season.
South African sources have indicated a similar deal is expected with Saracens.
Etzebeth, who has 53 Springbok caps, is regarded as one of the best forwards in world rugby, and the deal would be one of the biggest signings in Premiership history.
The deal would have to be approved by SA Rugby and, if it is completed, Etzebeth would act as injury dispensation for Saracens' former club captain Alistair Hargreaves, who was forced to retire this month because of concussion.
Etzebeth would link up with former Stormers Michael Rhodes and Schalk Burger at Allianz Park, and the move will raise further concerns in the southern hemisphere about the player exodus to clubs in France and England.
One of the leading administrators in South African, New Zealand, Australian and Argentinean Rugby (SANZAAR) told the BBC last week that the financial might of clubs in Europe was the "biggest challenge" to their unions.