Arsonists torched an area of grassland near the St Helens Linkway - endangering an area of woodland and local habitats.
Firefighters were called to the scene shortly after 5.30pm last Monday.
They used beaters to extinguish the blaze and remained at the scene for about half an hour.
Crew manager Ste McHale, of St Helens Community Fire Station, said: “The fire involved grassland near a local lake and near to a bit of woodland. It could have spread towards the woodland and destroyed local habitats.
“Deliberately starting fires on grassland and open spaces is arson and offenders will be prosecuted.”
He added: “If a fire appliance has to attend a grass fire that has been started deliberately it may delay the response to another emergency.
“Fires started on grassland near motorways and busy roads can cause smoke to drift across roads, creating an extra hazard for drivers. We investigate all fires we are called to.”