The Welsh Assembly Government has faced a second defeat in as many weeks by opposition parties uniting to delay a key government decision.
The motion passed by AMs delays the abolition of the language board
They have ordered Labour to postpone the merger of the Welsh Language Board into the assembly government.
The merger is due in April 2007 but Plaid Cymru want more clarity about the board's role after the proposed merger.
Labour has accused the opposition of "undermining the democratisation of Welsh public life".
The assembly's two independent AMs, John Marek, and the former Labour AM for Blaenau Gwent, Peter Law, voted in favour of Plaid Cymru's motion instructing the Welsh Assembly Government to postpone the merger.
Labour has been in a minority in the assembly since Peter Law became an independent earlier this year. The opposition now holds 31 of the 60 seats.
Last week, the opposition parties joined forces to make the Welsh Assembly Government re-draft its 2006 budget.
Labour has been defeated on two keys votes in as many weeks
Plaid Cymru hailed Tuesday's vote on the Welsh Language Board as a "victory for those people across Wales who wish to create a bilingual nation".
The party said the move would give the assembly time to draw up a satisfactory way for the board to monitor local education authorities' plans for promoting the language.
Assembly group leader, Ieuan Wyn Jones, said: "It is appropriate that there is an independent regulator who can ensure that whichever government is in power in Cardiff Bay, or in our local councils, (it) will promote the language."
However, the Welsh Assembly Government said its own legal advice is that nothing has changed, despite the motion being passed.
Culture Minister Alun Pugh said the language board's adjudication functions would be carried out by an independent regulator.
He said: "This motion has nothing to do with the Welsh language but with the Tories and Plaid jumping into bed with each other for a series of one-afternoon stands.
"They say they support the democratisation of Welsh public life and yet take every opportunity to undermine the process."