Evening charges, for leaving a vehicle from 5pm to midnight, are set to rise from £1 to £3 across all borough council-owned car parks.
At surface car parks such as Upper Mounts, the 60p for an hour tariff and £1.20 for two hours will be replaced with a new minimum charge of £2.
Anyone staying at surface car parks for between three and four hours will have to pay £4 - as opposed to £2.40 for three hours or £3.20 for four hours.
Councillor Brian Markham alerted the public to upcoming increases a week ago when the Liberal Democrats asked how the Conservatives planned to increase parking income by £625,000, as stated in their budget.
Today he said residents would struggle to understand why car parking charges are being increased when they could have a "devastating effect" on the town centre.
He said “I cannot believe that the proposed increases include a large increase in evening parking.
"The future of the town centre will rely on residents visiting restaurants and supporting the Royal & Derngate and this increase will do exactly the opposite”
The reports to cabinet also reveal that only 21 people participated in the budget consultation, which the Lib Dems say was because there was no indication of changes of this sort.
Papers to go before cabinet members at the Guildhall on February 20 say the aim of the changes is to encourage people to stay longer in Northampton town centre. If people have to pay £4 because they intend to stay for three hours, for example, they may decide to stay for five hours because the price is the same.
Council officer Rick O'Farrell said: "An important aspect is increasing the dwell time for visitors and minimising the public’s perception of receiving a parking enforcement ticket.
"It is important the council’s car parking provision supports the economic prosperity of the town centre and its parking tariffs are not only fit for purpose but they reflect market forces and are in line with regional pricing.
"The proposals contained within this report support the councils budgetary proposals and reflect the value of the parking service, while at the same time minimise the overall impact on car park customers."
The proposed charges are:
Surface car park changes
Monday to Friday
1hour 60p - £2
2hr £1.20 - £2
3hr £2.40 -£4
4hr £3.20 - £4
5hr still £4
All Day still £8
Evening £1 - £3
Overnight £2.50 - £3
Sunday
still £2
Multi-storey car park changes
Monday - Friday
1hour still free
2hr still free
3hr £2.40 -£4
4hr £3.20 -£4
5hr still £4
All Day still £8
Evening £1 - £3
Saturday
still £2
Sunday
still £2