The government has been defeated in the House of Lords in a vote about how Welsh assembly members are elected.
Peter Hain, Lord Davies (top); Lord Livsey and Lord Roberts (bottom)
Peers debating the Government of Wales Bill voted in favour of an amendment which maintains the current system of electing AMs to seats.
Labour had wanted to ban dual candidacy, preventing AMs from standing in both constituencies and on the regional seats list.
Welsh Secretary Peter Hain vowed to overturn the defeat in the Commons.
The lords voted for the amendment by 133 votes to 114.
Ministers had criticised the current system for "making losers [of constituency votes] winners" in the regional list.
However the Conservatives and the Liberal Democrats carried a Tory motion to preserve the status quo.
The Conservatives said the move was designed to benefit the Labour Party in Wales and should not be forced through.
Former Conservative minister Lord Roberts of Conwy said the government was being "partisan".
"The motive for the change is that Labour have no regional list members and their hold on the Assembly government is precarious," he said.
"It is their wish to protect their own sitting members from attack not only from regional list candidates but anybody else because they are in something of a fix."
Lib Dem peer Lord Livsey of Talgarth said: "The whole weight of informed opinion is in favour of, at the very least, retaining the present system."
Speaking for the government, Lord Davies of Oldham described the change as one which would affect all parties equally.
He said: "No party will gain or lose a single vote or seat in the Assembly as a result of this change.
"There are three ministers in the Welsh assembly government who are currently in marginal seats.
"They also will lose the safety net which the list system would otherwise have provided."
Welsh Secretary Peter Hain said the government would overturn the defeat when the bill came back before the House of Commons.
"The ban on dual candidacy was an explicit manifesto commitment put before the people of Wales in the 2005 general election," he said.
"The government is 100% committed to ending the abuse of the assembly's electoral system, putting the voters back in charge by stopping rejected constituency candidates getting into the assembly by the back door."
Mr Hain added: "We will reverse this decision by the unelected House of Lords when the Government of Wales Bill returns to the democratically elected House of Commons."