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 the Welsh Assembly

Joint statement from Plaid Cymru/SNP Group

According to joint leaders Meirionnydd Nant Conwy MP Elfyn Llwyd and Alex Salmond, the alliance will enable them to "punch their weight more effectively".

The leaders said they would agree to differ on some policies including independence, which the SNP supports but which Plaid has branded an "outdated concept".

The nationalists will rank behind the Conservatives (166 seats) and the Liberal Democrats (52) and ahead of the four smaller opposition parties in Northern Ireland.

In a joint statement the parties said: "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 the Welsh Assembly.

Lord Elis-Thomas: Worked with SNP before

"This 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."

Plaid Cymru and the SNP have worked together at Westminster since the late 80s when Plaid were led by Lord Dafydd Elis-Thomas.

They collaborated then on parliamentary issues, nominating one speaker for debates and at question time to increase their chances of being called by the speaker.

The arrangement never formally ended but faded away after devolution in 1999, when the SNP MPs - and most of the Plaid ones - began spending more time away from Westminster.

At the general election Plaid Cymru increased its share of the vote but while it captured Carmarthen East and Dinefwr from Labour, it also lost the Anglesey seat of Ynys Mon.

The SNP held five Westminster seats, but was the only party in Scotland to lose a seat when the Tories took the Galloway and Upper Nithsdale constituency.

They also saw its vote fall by more than 150,000 from its 1997 general election level.