Mr Murphy rejected the plea from Plaid

Currently, the assembly - which controls an annual budget of �10bn and is responsible for key public services - lacks the power to raise taxes or make its own laws, as the Scottish Parliament can.

When it was established in 1999, the Labour party was careful to limit the fledgling body's powers.

The Welsh secretary, who represents Wales's interests at the Cabinet table in London, said the constitutional debate was "a turn-off" for most voters, who were more concerned with the key issues of schools, hospitals and the environment.

Mr Murphy decided to speak out after the assembly was branded a "failure" by Plaid Cymru leader Ieuan Wyn Jones.

Delivering the annual Institute of Welsh Politics lecture in Aberystwyth earlier this week, Mr Jones said the current settlement was fatally flawed and would benefit from being more closely modelled on the Scottish Parliament.

On Friday, Mr Jones said Mr Murphy and the "dinosaur tendency" in the Labour party were being left behind in the devolution debate.

"A growing number of Labour AMs are coming out to support a law-making Parliament, because they are frustrated at the Assembly's failure to deliver in key policy areas," he said.

Welsh Conservative leader Nick Bourne, however, dismissed the Plaid calls, saying the assembly had to properly use the powers it has.

Public lecture

Mr Murphy is also determined to face down calls from within his own party for the assembly to gain legislative functions.

The Labour assembly member for Clwyd West, Alun Pugh, recently added his voice to those calling for a full Welsh parliament.

He will be giving a public lecture on the subject to students at the University of Glamorgan on 5 December.

And new Labour MP for Aberavon, Hywel Francis, said he was not opposed to giving the institution more power, but he claimed another referendum should be held before the constitutional settlement is changed.