Since 2002, the UK has lost nearly a third of its population and it is thought there are fewer than a million left.
This means hedgehogs are declining at a rate of around five per cent a year.
Reasons include loss of hedgerows, pesticides, and fencing between gardens which restricts their movement.
Chichester District Council is supporting Hedgehog Week, which starts on Monday.
Cllr Roger Barrow, who heads environment at Chichester District Council, said: ‘The British Hedgehog Preservation Society is encouraging people to record their sightings of hedgehogs.
‘We are supporting this, by asking people to let the society know when they come across a hedgehog so that we can establish how many there are and determine where they are.’
People are being asked to check for hedgehogs before cutting lawns. People can report sightings by calling 0207 4984533 or visit hedgehogstreet.org