The Prime Minister told MPs that no-one should be "religious on timing" when it came to the £2.4bn Universal Credit scheme.
Labour last night said the Coalition's promises on the project had been "shot to pieces".
Designed to replace a whole range of benefits, Universal Credit is at the centre of the Tory-Lib Dem Coalition plans to ensure that work always pays.
But the scheme has been beset by a number of complications and delays.
A highly critical report by the National Audit Office this week warned that the project still faced major difficulties.