Official UK information
The Water Industry Act 1991
Under s.70 of this Act, it is an offence, subject to a "due diligence" defence, for a water undertaker to supply water unfit for human consumption.
The Private Water Supplies Regulations 2009 or 2007 in Scotland, 2010 in Wales and Northern Ireland)
These make it a duty on Local Authorities to serve notices where water is “unwholesome” or a potential health risk. Cryptosporidium in the water will cause the water to be both. Notices are described in Regulation 18 for unfit water and in Section 80 of the Water industry Act 1991 for unwholesomeness.
Local authorities also have a duty under the Regulations to monitor private water supplies (except owner-occupied single domestic properties) for contaminants. However the monitoring does not include Cryptosporidium specifically. Possible contaminants identified during a statutorily required risk assessment should be included in any monitoring as specified in the regulations and Cryptosporidium is often identified as likely to be present. Many would argue however that the faecal indicators that are specified in the regulations (including Clostridium) are likely to indicate the possible presence of Cryptosporidium and that Cryptosporidium is so unlikely to be found during random sampling and is so expensive to find that it is not routinely worth looking for.