"As I am speaking to you right now he has been handed over to the Zimbabwean police," Biti's lawyer Gilbert Phiri told AFP, charging that his client had been deported in defiance of a court ruling.
A veteran figure in the opposition Movement for Democratic Change (MDC), Biti fled to Zambia on Wednesday, amid what his party says is a crackdown over its claims that last week's landmark elections in Zimbabwe were rigged.
The UN's refugee agency said it was "gravely concerned about reports of the forced return" of Biti, an internationally respected finance minister in Zimbabwe's 2009-13 power-sharing government.
"Forcibly returning refugees and asylum-seekers to their country of origin is a serious violation of international refugee law," the UNHCR said in a statement.
It called on Zambia to "investigate this reported incident urgently."
Zambia had swiftly rejected Biti's asylum claim and said he would be sent back across the border, but the 52-year-old's legal team filed a challenge keeping him there until the case was reviewed.
However, Zambian government spokeswoman Dora Siliya said the legal order had not been received until it was too late.
"His asylum was denied on the basis that in his country there is no breakdown in the rule of law," she said.