PLAID Cymru leader Ieuan Wyn Jones said today that the Liberal Democrats cannot be relied upon to do what is best in a hung Parliament.

He said the Lib Dems "bottled’’ an opportunity to join a coalition government in Wales after inconclusive Assembly elections in 2007.

The Lib Dems drew up a so-called rainbow coalition with Plaid and the Tories which would have made Mr Jones First Minister, but the deal fell apart when Welsh Lib Dem chiefs split on it.

Mr Jones's broadside follows attacks against the Lib Dems by the Conservatives and will be seen as an attempt by the nationalists to counter the Lib Dems' opinion poll surge.

There are signs the Lib Dems' poll bounce following Nick Clegg's performance in the televised prime ministerial debates could be partly at Plaid's expense.

A YouGov poll for ITV Wales last week showed the Lib Dems had gained 17 points in a month, putting them second behind Labour, while Plaid had slipped five points.

Plaid has put the prospect of a hung, or balanced, Parliament front centre in its election campaign, saying it would team up with the SNP to seek a better deal for Wales in exchange for supporting a minority government.

Plaid and the Lib Dems are going head to head in the mid-Wales seat of Ceredigion. The Lib Dems took the seat from Plaid with a majority of 219 at the last election.

Mr Jones said: "Their (the Lib Dems') politicians are as unreliable as their policies and at a time of economic instability the UK cannot afford to rely on a party which will always put their own narrow interests ahead of doing what's best for our communities.

"With a balanced Parliament now the most likely outcome in this election, there is a real opportunity to ensure meaningful and constructive co-operation between the parties and between the nations within the UK.

"Working across parties - with the public sector, business, industry and the trade unions - a new model of political and social partnership could be created in order to tackle the deficit and ensure recovery.

"In such difficult economic times, and with faith in politics at such an all-time low, the people expect nothing less.’’ The Deputy First Minister added: "Plaid has always said it will work in the best interests of Wales, which is what we are doing in the National Assembly.’’