Background and Purpose: Currently there are no licensed vaccines and limited antivirals for the treatment of COVID-19. Heparin (delivered systemically) is currently being used to treat anticoagulant anomalies in COVID-19 patients. In addition, in the UK, nebulised unfractionated heparin (UFH) is currently being trialled in COVID-19 patients as a potential treatment. A systematic comparison of the potential antiviral effect of various heparin preparations on live wild-type SARS-CoV-2, in vitro, is thus urgently needed. Experimental Approach: A range of heparin preparations both UFH (n=4) and low molecular weight heparins (LMWH) (n=3) of porcine or bovine origin were screened for antiviral activity against live SARS-CoV-2 (Victoria/01/2020) using a plaque reduction neutralisation assay and Vero E6 cells. ND50 values for each heparin were calculated using a mid-point probit analysis. Key Results: UFH had potent antiviral effects, with ND50 values of 12.5 and 23 μg/ml for two porcine mucosal UFH tested. Bovine mucosal UFH had similar antiviral effects although it was ~50% less active (ND50, 50-75 μg/ml). In contrast, LMWHs such as Clexane and Fragmin were markedly less active by ~100-fold (ND50 values of 2.6-6.8 mg/ml). Conclusions and Implications: This comparison of a panel of clinically relevant heparins, including the UFH preparation under trial in the UK, demonstrated that distinct products exhibit different degrees of antiviral activity against live SARS-CoV-2. Porcine mucosal UFH has the strongest antiviral activity followed by bovine mucosal UFH, whereas LMWHs had the lowest amount of antiviral activity (by 100-fold). Overall the data strongly support further clinical investigation of UFH as a potential treatment for patients with COVID-19.