Varndell is the most prolific try scorer in the history of the Aviva Premiership having scored 92 tries during spells at Leicester Tigers, Wasps and Bristol.
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Wayne Pivac has been keen to bring in back three cover after wingers Tom Prydie and Johnny McNicholl were ruled out for the remainder of the season, while Steffan Evans and Leigh Halfpenny are away on international duty.
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Tom Grabham is also a long-term absentee, Rhys Jones is also sidelined, while Morgan Williams is on sevens duty. The 32-year-old has won four English caps, the last coming in 2008 in a summer Test against the All Blacks.
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The deal is until the end of the season and Varndell will be registered in the Scarlets’ European Champions Cup squad ahead of their home quarter-final against La Rochelle on Good Friday.
 Bristol head coach Pat Lam said: “Scarlets head coach Wayne Pivac and I have a good relationship and they’re short of back three players leading into an important period of their season.
“Tom needs gametime, so it was the right decision for both parties. Tom has served the club well over the past three seasons, but he wants to play regularly and he has that opportunity in the PRO14 with Scarlets.
“Tom is out of contract at Bristol at the end of this season, so this is a good opportunity for him to showcase what he can do at another club. We wish him all the best and thank him for his efforts in the Bristol shirt.”
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