Shell is to offshore over 3,000 UK IT jobs and retain only a small core of staff in its UK offices.
Unite, the union previously known as Amicus, is now threatening Shell with legal action over the redundancy terms offered by the company.
IT staff are being offered a maximum payoff of £50,000, a quarter of the £200,000 offered to Shell's oil rig workers.
David Fleming, national officer at Unite, said: "We will take a zero-tolerance approach to any company that makes compulsory redundancies as a result of offshoring.
"Ministers and MPs have turned a blind eye to the long-term effects of offshoring on the economy, but as unemployment creeps up in their constituencies they will have to take notice."
The plans emerged after an email notice sent out just before Christmas was forwarded to an anti-Shell website.
Michiel Brandjes, Shell's company secretary, said on the site that the email is "authentic".
"It is not a matter of a leak. Shell is currently in the midst of finalising contract negotiations with selected suppliers to outsource IT infrastructure," he wrote.
"This has been public knowledge since the announcement of a study to outsource a substantial part of IT infrastructure to a few selected suppliers in October 2005.
"This decision has been the outcome of a transparent process involving employees and relevant works council."
The plans have been criticised by the National Outsourcing Association (NOA). "What is of interest here is how Shell seems to treat its IT workers in comparison to its oil rig workers," said Martyn Hart, chairman of the NOA.
"Shaving 75 per cent off the redundancy package looks like a fiscal kick in the teeth. And it is not just a monetary issue.
"This action could undermine the importance of IT's role within the company. IT is fundamental to the functioning of all organisations, and Shell and other companies would do well to remember that."
