Prime Minister David Cameron says the referendum on whether the Welsh assembly will get more powers should be held in 2011.
The Welsh Assembly Government was strongly critical, and said it had no warning of his announcement.
The assembly government had asked Welsh Secretary Cheryl Gillan to hold the ballot next autumn.
She had yet to respond, but said too little work was done by the previous Labour government to hold it then.
Mr Cameron told MPs: "What we're going to do is allow the referendum to go ahead that was actually rather held up by the last government, so yes, a date will be named for that referendum and I believe it should be held next year and I believe there should be a free and open debate in Wales for that to happen."
He sidestepped a question from the Wrexham Labour MP Ian Lucas on whether he would support a "yes" vote as he said he did not have a home in Wales.
The Electoral Commission says a 10-week period is needed to ensure the referendum question can be easily understood by voters, which would rule out an autumn poll.
Mrs Gillan said she would not rule out an autumn referendum but added: "You can do the maths yourselves, it's the First Minister [Carwyn Jones] who hasn't fully appreciated the maths."
Mrs Gillan said she found key elements of the preparations for a referendum "unfinished business" on her desk when she took office.
That was denied by her Labour predecessor, Peter Hain, who insisted the work he carried out on further devolution would have allowed a referendum to take place this autumn.
In a statement, the assembly government said: "We are disappointed about the issue of timing, as we understood the Electoral Commission could amend its timetable to accommodate a referendum this autumn.
"We are also dismayed to have been told of this announcement via the media and not through the formal channels of government.
"Indeed, we are surprised this announcement was made before we had received any communication from the secretary of state [Mrs Gillan].
The Liberal Democrats share power with the Conservatives in the UK government, but the party's leader in Wales, Kirsty Williams, said an autumn poll was still achievable.
"While I recognise that the time scale to hold a referendum in 2010 is becoming tighter, it is still possible for us to have a vote on the future of the assembly this Autumn.
"If Peter Hain had pulled his finger out during his time as Labour's secretary of state for Wales, we wouldn't be having this discussion."
Plaid Cymru claimed the timing of the vote was not a matter for the prime minister.
The party's Deputy Assembly leader, Helen Mary Jones said, "We shouldn't be surprised at Cameron's behaviour, trying to dictate the date of a Welsh referendum from Westminster but we need to be clear that it isn't for him to decide."
"It is also another example of how little influence the ConDem leaders in Wales, Kirsty Williams and Nick Bourne, have over their London bosses.
"Both have stated a strong preference for an autumn referendum. This preference has been ignored by David Cameron and Nick Clegg."
Mr Cameron's announcement came after the Queen's Speech setting out the coalition UK government's first programme.