Mu Sochua faces a defamation charge from Prime Minister Hun Sen
Cambodia's National Assembly has voted to strip parliamentary immunity from two opposition members.
The vote means the two politicians, Mu Sochua and Ho Vann, can be prosecuted on defamation charges.
The UN's human rights organisation has criticised the government's increasing use of the courts against its critics.
The government has defended its use of the courts, saying in one case a critic had attempted to cause "chaos and confusion" with inaccurate remarks.
The ruling Cambodian People's Party has a large majority, so the vote against the two members of the opposition Sam Rainsy Party was never in doubt, says the BBC's Guy DeLauney in Phnom Penh.
The two politicians are among at least six critics of the government who have been the subjects of recent legal action.
Mu Sochua had originally launched a defamation suit against Prime Minister Hun Sen. That was dismissed and the prime minister counter-sued.
Ho Vann is being sued for defamation by senior military officials, over allegations that he insulted the quality of their university degrees.
The UN's human rights office in Phnom Penh recently issued a statement expressing concern about the use of defamation and disinformation laws to silence government critics.
But the government has consistently defended its human rights record, pointing to the success of several democratic elections since the return of peace in the 1990s.