A Public and Commercial Services (PCS) union spokesman said, "Strike action cannot be ruled out in the future if warnings are not heeded."
The union warned that plans to privatise the department's IT could threaten up to 1,000 jobs.
Defra plans to advertise the outsourcing contract later this year, despite the fact that it is currently looking for an IT director and has no clear IT strategy.
Geoff Lewtas, PCS senior national officer, said, "Defra is determined to flog off its skilled and dedicated IT staff to the private sector and risk decimating its existing service."
A spokesman for Defra said no redundancies are planned at this stage. "We are in close contact with our IT staff and the unions. We are committed to taking our staff through any forthcoming changes."
The government official said the department is creating a new IT strategy, which it will complete by the end of this month. "No final decision about the department's long-term IT strategy has yet been made," he added.
The Commons environment, food and rural affairs select committee recently said that Defra should postpone outsourcing until it has an IT strategy and an IT director.
In January, the Office of Government Commerce conducted a strategic assessment of Defra's outsourcing proposal. It warned ministers that it would be a very high-risk project and identified a shortfall in skills and resources.