Sir Alan Budd, back row, left, was an original member of the Monetary Policy Committee
The Conservatives' proposed watchdog for the nation's finances is to be set up in shadow form under the leadership of ex-Treasury adviser Sir Alan Budd.
The Office for Budget Responsibility is being set up early so it can publish forecasts ahead of the emergency Budget planned if the Tories win the election.
Shadow chancellor George Osborne said it would hold them "to account for promises made to the British people".
Sir Alan told the BBC it would "keep the chancellor's feet to the fire".
The Conservatives, who announced plans for the Office for Budget Responsibility in 2008, have said they will hold an emergency budget within 50 days if they win the election.
Sir Alan is a former chief economic adviser to the Treasury and was one of the original members of the Bank of England's Monetary Policy Committee in 1997.
He told BBC Radio 4's Today programme he had been asked to oversee the process of establishing the new body.
He said that while it would not have powers to overturn decisions it would be "inescapable" - with the chancellor forced to answer any criticism from it in Parliament.
Mr Osborne said the party was "promising today that by time of our first budget we will have up and running an Office for Budget Responsibility that will hold a Conservative government to account for the promises it makes to the British people".
"This is in order to avoid the soaring taxes and higher interest rates that a debt crisis would lead to."
The announcement comes the day before the pre-Budget report in which Chancellor Alistair Darling is expected to give more details of Labour's plans to halve the budget deficit within four years.