By Adrian Browne BBC Wales political reporter

Former diplomat Sir Emyr Jones Parry is chairing the convention

Holding the poll on further powers, within four years, is a key commitment of the parties' coalition agreement.

Labour members of a committee deciding the rules for a convention on the issue want it to be neutral on whether or not more devolution is needed.

Plaid wants the convention to make the case for a full Welsh parliament.

It emerged on Monday that Labour and Plaid Cymru see the convention in radically different ways before a committee of AMs and MPs deciding its terms of reference met in Cardiff Bay.

BBC Wales understands Labour wants the convention to concentrate on commissioning research on public opinion rather than persuading voters one way or the other.

Plaid not only wants an investigation of new areas in which the assembly could gain powers as well as the need for extra assembly members, but also wants the convention to be essentially a campaigning body.

A Plaid-style convention would seek to build a consensus for a Welsh parliament, Labour's would confine itself to evidence gathering.

Currently, in order to make laws in devolved areas, the assembly government first seeks extra powers from the UK government.

If a referendum were won, Wales could pass its own laws on matters such as health, education and transport without consulting Westminster.