When must a UK business file a suspicious activity report?

As soon as practicable after knowing or suspecting, or having reasonable grounds to know or suspect, that another person is engaged in money laundering, where the information came in the course of business in the regulated sector. The report goes to the National Crime Agency through the nominated officer, and failing to report is itself an offence carrying up to five years' imprisonment. The test is objective, so the absence of actual suspicion is no defence where the grounds were there.