The devolved nationalist parties are set to join in a parliamentary merger creating the House of Commons third largest opposition grouping.

Plaid Cymru and the Scottish National Party will today combine forces, to the left of Labour, forging an historic joint parliamentary group overtaking the Ulster Unionists as a party of opposition.

The new Westminster force will be led by the SNP's Alex Salmond and Plaid's Elfyn Llwyd, who claim that the new political formation allows them to "punch above our weight".

"As the third largest opposition group we will have more opportunities to contribute during question time, in key debates in the chamber and through stronger representation on House of Commons committees.

"By working together, we can make the voice of Wales and Scotland heard on issues as diverse as protecting public services, the rural crisis and the future growth of the Scottish Parliament and Welsh Assembly," said a joint statement.

The combined nationalist group is set to take up opposition to Labour's left on the issue of privatisation and will press for further devolved powers to the Scottish parliament and the Welsh assembly.

"Our new group will be built on a determination to provide the strongest possible opposition to the government's privatisation agenda. the new initiative will allow us to punch above our weight allowing the Plaid Cymru/SNP Group to lead the opposition to the government's assault on public services in our nation," said the new group.

Llwyd conceded that political differences existed between the two parties but described Welsh and Scottish nationalists as "pretty similar".

Scottish and Welsh Liberal Democats have attacked the move.

Welsh and Scottish spokesmen for the Lib Dems, Lembit Opik and Michael Moore, questioned the abilty of the new grouping to represent Welsh people.

"Plaid Cymru made a lot during the election about them not taking orders from elsewhere and standing up for Wales. Now it seems they will stand up for Wales as long as it doesn't upset the Scots.

"They now have to answer questions as to who will respond on behalf of their group in parliamentary debates Elfyn Llwyd or Alex Salmond? Either Wales or Scotland will be denied a voice in these debates," they said.

With Labour's huge majority concentrating the minds of opposition MPs, the pattern of the parties will now be the Conservatives - 166 seats, the Liberal Democrats - 52 seats, the Plaid/SNP group - nine seats, Ulster Unionists - six seats, Democratic Unionists - five seats, Sinn Fein - four and the SDLP's three seats. The nationalist MPs will meet weekly.