Armed forces minister Bill Rammell said extra work on the deal meant a delay to plans to make a decision in the spring.
Vale of Glamorgan MP John Smith said there could be "no doubt whatsoever" at UK ministers' commitment to the scheme.
Planning permission for the DTC was granted by Vale of Glamorgan council in September last year, although the final say on the go-ahead rests with the Ministry of Defence (MoD).
Construction was due to begin late this year, with the college billed to provide specialist engineering, communications and information systems training to all the UK's armed forces, bringing them together in one location on new premises.
The UK's armed services will have specialist training at the college
Mr Rammell said plans for the college had "advanced considerably" but the work involved had "exceeded assumptions".
He said: "It is not uncommon for a project of this complexity and size to experience delays, but the project is under constant review to ensure it meets value for money requirements.
"The benefits the project will deliver are very considerable and include real savings against existing defence budgets over the next 30 years."
Mr Smith, who is standing down as an MP at the general election, welcomed "the firm timetable".
He said: "The St Athan project is making good progress and we are now approaching the home stretch.
"I want people here in the Vale of Glamorgan to know that I will go the extra mile in the time I have left in parliament to facilitate the start of this record investment which has the potential to transform the fortunes of the Vale and the Welsh economy for the better."