Neither the Tories nor Labour is offering a "properly credible prospectus" in their General Election manifesto spending plans, a leading economic think tank has said.

The Institute for Fiscal Studies (IFS) said it was "highly likely" that a Conservative government would end up spending more than the party's manifesto implied - meaning either taxes or borrowing would have to rise.

It said that Labour would not be able to deliver on its promise to raise investment levels by £55 billion a year as the public sector does not have the capacity to "ramp up" that much that quickly.

It was "highly likely" that a Labour government would have to find other tax increases beyond those it has announced for big business and the better off if it was to raise the extra £83 billion a year in additional revenues it wants.

"In reality, a change in the scale and the scope of the state that they propose would require more broad-based tax increases at some point," said IFS director Paul Johnson.

Mr Johnson said that the chances of the Conservatives being able to hold spending down over the course of a five year parliament in the way that they proposed appeared to be "remote".

"Why have they been so immensely modest in their proposals? Because to do otherwise would either mean resiling from their pledge to balance the current budget or would mean being up front about the need for tax rises to avoid breaking that pledge," he said.