Transport minister Tom Harris stated in the Commons that a 'considerable amount of work was still to be done' before the redevelopment could be given the go-ahead.
His statement came amid warnings from Birmingham MPs that action must be taken, because the station already cannot cope with the number of passengers passing through daily.
The proposals for a £500 million redevelopment would ease congestion that often leads to the station's closure.
Edgbaston MP Gisela Stuart said: 'It is a station which, to pick up on the parlance of the day, is not fit for purpose. More passenger capacity is desperately needed at New Street.'
The admission that a decision on the station's future will be delayed follows criticism by a local authority councillor who claimed the station lacked a 'wow factor' and the designs would need to be reconsidered.
A spokesman for developer Network Rail said government decisions were always likely to take a long time.
He added: 'We anticipated these delays and scheduled them into our plans. The redevelopment will cost an awful lot of money - £380 million of public funds - so it is understandable that it will take time.'