Microsoft, which is phasing out NT4 in favour of Windows 2000, announced the move after a last-minute rush of candidates stretched testing centres beyond capacity.
IT professionals will now have until 28 February to sit NT4 Microsoft Certified Professional exams, but user groups claim the deadline is too short for most IT departments.
"Our members feel very strongly about the premature phasing out of the certificate. There is still a remarkably strong following around Windows NT and there will be for the foreseeable future," said Simon Moores, chairman of Research Group.
"This is an example of Microsoft trying to coerce its customers into upgrading to Windows 2000," he said.
Microsoft defended the move, saying it would ensure that IT directors would have a supply of staff with the latest skills.
The company is also offering a new examination that will test the skills needed to support NT4 in a networking environment, in an attempt to soften the blow.