It happened just after 8.20pm on Saturday in Leominster Road in Paulsgrove, and involved a silver Volkswagen EOS Sport.
The 19-year-old was taken to Southampton General Hospital and was said to be in a serious condition by police on Sunday.
Police have now said the teenager is in a critical condition.
A spokeswoman said: ‘Enquiries are ongoing to establish what happened. Anyone who saw the collision should call 101 quoting 44210230490.’
Portsmouth City Council and Voi have confirmed the escooter was not one of the escooters taking part in the city trial – suggesting it was a private escooter.
Police advice on private escooters states: ‘A privately owned escooter can only legally be used on private land and with the permission of the land owner.
‘In legislation, an escooter is classed as a powered transporter and are treated as a motor vehicle and so fall under the Road Traffic Act. This means that they are subject to all the same legal requirements as motor vehicles; MOT, tax, licensing, insurance and specific construction regulations.
‘As escooters can’t currently meet these requirements, riding a privately-owned e-scooter on a public road, pavement or other public area is a road traffic offence.’