In a letter to CAS seen by Reuters, lawyer Sarah Ochwada said several obstacles would hinder her from defending Jeptoo and the International Association of Athletics Federations (IAAF) and CAS had no sympathy for the athlete's situation.
She added that the IAAF would use Thursday's hearing to ask for Jeptoo's ban to be increased from two years to 12 and said the runner was not capable of representing herself.
Jeptoo was suspended for two years in October 2014 after failing an out-of-competition test for the banned blood-booster EPO, becoming the most high-profile positive test from Kenya, which has seen a raft of doping cases in the past two years.
"Ms. Jeptoo is facing an extension from a standard two-year ban to a disproportionate 12-year ban," Ochwada, who was working for the athlete for free, wrote in the letter.
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"Due to her limited ability to communicate in English and her level of education she is not competent to understand let alone to prepare legal documents or argument for herself."
Ochwada also said Jeptoo had no access to facilities for a video conference for the hearing and that Thursday was a public holiday in Kenya, making it difficult for her to summon witnesses.
"It is a sad day for us as it is clear from the foregoing that the IAAF and the CAS have no sympathy for Ms Jeptoo or for us as her legal representatives. It is for the above reason that we can no longer proceed as pro-bono counsel in this matter," said the letter.
"We therefore invite the CAS to appoint new pro-bono legal counsel to assist her to navigate the complexities of this case."
CAS said the hearing would still go ahead on Thursday.
"Ms. Jeptoo is advised that the hearing will proceed as scheduled on 7 July 2016," CAS counsel Brent J Nowicki said in a letter seen by Reuters.
The runner, who won back-to-back Boston and Chicago titles in 2013 and 2014 plus the Boston event in 2006, has denied doping.
(Editing by Clare Fallon)
