Google sued for using the NHS data of 1.6 million Britons 'without their knowledge or consent'

Google is being sued over its use of confidential medical records belonging to 1.6 million individuals in the UK. The company's artificial intelligence arm, DeepMind, received the data in 2015 from the Royal Free NHS Trust in London for the purpose of testing a smartphone app called Streams. The claim is being brought by Andrew Prismall in a representative action in the High Court. It alleges that Google and DeepMind "obtained and used a substantial number of confidential medical records without patients' knowledge or consent". Why did Google get access to patient records?