The investigators have developed a short ACT-based treatment (PACT) for physiotherapists to deliver and aim to compare it with usual physiotherapy care. The investigators will recruit 240 people with CLBP from three hospitals in South East London. They will be randomly divided into two groups, with half receiving PACT and the other half ordinary physiotherapy. PACT consists of two hour long sessions and one follow-up phone call, meaning fewer hospital visits for patients and more time during sessions for individualised treatment. It aims to encourage people to focus less on getting rid of their pain and more on moving forward with what is most important in their lives. The investigators will compare PACT with usual physiotherapy to see which is most successful at improving people's ability to function and their quality of life and which approach helps them to manage their back pain best in the long term. If PACT is effective, the investigators believe it could reduce the considerable burden of CLBP to patients, the NHS and society.