The fate of a proposed £14bn defence training academy in south Wales is expected to be learnt, as UK ministers unveil future military spending.
The Defence Technical College (DTC) is set to create more than 2,000 jobs, providing training for the armed services.
Unions are lobbying MPs at Westminster as the strategic defence and security review (SDSR) is unveiled.
It will decide the future shape of the armed forces.
The fate of the DTC project at St Athan in the Vale of Glamorgan has remained uncertain as it has seen a series of delays, with construction due to begin late this year.
The academy is expected to offer specialist engineering, communications and information systems training to all the UK's armed forces, bringing them together in one location on new premises.
The project was originally envisaged as centralising training for the three armed forces, and funded by the Ministry of Defence (MoD) as a private finance initiative (PFI).
The consortium behind the scheme, Metrix, has said around 2,200 jobs would be created, most in security, cleaning and catering, with about 800 workers employed during a four-year construction period.
The union Unite, which includes the former engineering and electrical union Amicus, claims the SDSR will be a "body blow" for Ministry of Defence jobs.
Kevin Coyne, Unite's national officer for the MoD, said the SDSR came as 170 staff at the military aircraft maintenance site on the base were expected to be made redundant by Christmas.
He said: "It does not look particularly positive. Our view is that it would have been a stop-gap.
"How can we build our way of of the recession if we are scrapping apprenticeships?"
Welsh union members are travelling to Westminster to lobby MPs ahead of the SDSR, with Unite, Unison and the PCS all taking part.
Welsh MPs and politicians have campaigned for the military training academy to proceed.
Vale of Glamorgan MP and AM for South West Wales, Alun Cairns, said: "I'm obviously hopeful that the project will go ahead.
"It's the right military solution and it will save money over 30 years. However, the Treasury and the MoD are looking for savings over the next three to five years.
"None the less, this fits into the MoD's plan of 'spending to save'".
Mr Cairns said he and other MPs were meeting Defence Secretary Liam Fox later on Tuesday to learn more SDSR details.