On the day the Conservatives unveil their full election manifesto, Labour will launch a strategic attack on the Tory's tax and spending plans.

The Labour Party will release a Treasury-costed estimate that the Tory manifesto contains commitments to an extra £15.7bn of spending by 2007-08.

This is the date by which the shadow chancellor, Oliver Letwin, has promised to reduce spending against Labour plans by £12bn.

It is the first time Labour has produced an estimate of the extra spending commitments of shadow Cabinet members, but the party believes the publication of figures holds the key to undermining Tory credibility.