The company, which was founded in 1999 to sell country clothing at outdoor events, now has more than 70 shops and is valued at around £55m.
It hired corporate advisory firm Rothschild in November to advise on selling a minority stake in the business to raise funds for growth.
In November the group sold a chunk of its young fashion brand Rampant Sporting to an Indian investor to concentrate on its core business.
Among the other private equity firms understood to be mulling an investment in Joules are LDC, the buyout division of Lloyds Banking Group. Graphic Capital is said to have pulled out of the running.
It is unclear how much of his majority stake Tom Joule, the founder and chief executive, intends to sell.
Joule bought 80 per cent the business from his father, Ian, in 1999 and turned it from an equestrian-based label into a lifestyle brand.