Lib Dem group to put price on rivals' promises

Detailed studies of the tax or public borrowing impact of economic, environmental and social policies are to be used by the Liberal Democrats to challenge the Government and Labour to come clean over the costs of manifesto promises in the run-up t o the next election.

The party's costings workings group, composed of economic and financial specialists working at arm's length from the policy-making wing, was set up on the instructions of Paddy Ashdown, the Liberal Democrat leader, last autumn.

A further six to nine months' work is to be carried out before detailed costings of all Liberal Democrat policies are made public.

However, the party has already exploited progress so far with an analysis of spending promises made by Tony Blair and Shadow Cabinet members. It warned that income tax would rise by 5p in the pound or borrowing by £12bn.

The idea of a Liberal Democrat "red book" to be read alongside the real thing produced at each Budget follows informal reports emanating from the Treasury that the party's analysis of 1992 manifesto commitments was the only one that was "near accurate".

Party sources admit that last autumn's Liberal Democrat shadow Budget was only a "preliminary stab" in a far more sophisticated exercise, and that much more intensive work needs to be done by the group. It includes David Laws, a former city economist; Philip Vince, a long-standing former Liberal party adviser on social security; Maurice Fitzpatrick, a tax specialist; and Sir William Goodhard, the QC and chairman of the Federal Policy Committee, which will make final policy choice in conjunction with theparty conference.