The group claim parking tickets issued by borough council wardens omit the date of issue and the date of the contravention, both of which are needed for them to comply with the relevant Road Traffic Act.
They have been emboldened by last week's High Court ruling of Justice Rupert Jackson in a similar matter which went against Barnet Council.
The court ruled that the tickets issued without dates were invalid, which can now be considered in parking fine appeals.
A legal notice was handed to Northampton Borough Council legal officers at the Guildhall on Friday by group member John Stanton of Glasgow Street, St James.
Mr Stanton said: "The council has been warned before by the National Parking Adjudication Service when they sent a memo to all councils last May saying that if they did not comply, each did so at its own risk
"Northampton Borough Council, even though they have been warned by NPAS and have been emailed by us, have buried their heads in the sand and continued to issue what we see as illegal parking tickets.
"Now the Barnet Council case has set a legal precedent, I think it gives us a much better chance of getting Northampton Borough Council to admit they have been wrong.
"We estimate the borough council must have issued several million tickets without these proper dates on and our ultimate aim would be to claim that back."
The writ gives the borough council legal department 18 days to respond to the claims before the group attempt to start legal proceedings.
Mr Stanton told the Chronicle & Echo he would be willing to see the matter through to the courts.
A spokeswoman for the borough council confirmed that a member of the borough council legal department had received the writ from Mr Stanton on Friday.
nick.spoors@northantsnews.co.uk