To avoid or mitigate risks associated with introduction of non-indigenous species, it is necessary to monitor and understand the quantity and origin of foreign and domestic ballast water discharged into territorial waters. It is not yet compulsory to report a ships ballast water operations in any ports within the UK. 38 ports have been contacted and their shipping operations from 2003 to 2008 have been collected. Despite the diverse nature and format of the data, it has been possible to develop a correlation between the amount of cargo and ballast water operations for every port and hence estimate the potential amount of ballast water discharged. The proposed methodology has been used to estimate the total amount of ballast water discharged in UK waters and at individual ports. Based on this estimation, 20 UK ports with the highest amount of imported ballast water from foreign origins have been identified.