In a statement released on Sunday, Lincolnshire police confirmed they were treating the incident as criminal damage.
A spokesman for the force said on Sunday: “Shortly before 11.15pm yesterday, we received reports of a person shown by CCTV acting suspiciously near the site.
“Officers attended and found that graffiti had been spray-painted on the barriers surrounding the statue, no damage was thought to have been caused to the statue.
“This is being treated as criminal damage and there is an ongoing investigation.”
Peter Tully, who shared photos of the vandalism, described the statue as “controversial” and told Lincolnshire Live: “Support for the statue is 50 to 50 in Grantham, but across the country it is probably 70-30 against. “
Wayne Dean, a 52-year-old Grantham resident, added: “I’ve never liked it, but this vandalism is out of service. It’s a shame. I don’t have to.”