The efficacy of CBT for loneliness in children and young people will be evaluated through a randomised multiple-baseline single case experimental design. The design consists of AB+ post-intervention, where A is the baseline phase, B is the intervention phase and then a post-intervention phase. In the SCED approach participants are repeatedly assessed on at least one independent variable, in this case self-reported loneliness on the Three-item Loneliness Scale, across each phase of the intervention. This repeated measurement and within subject replication is then used to test the effects of the intervention for individual participants.

The construction of this SCED trial and the reporting of the results will be in accordance with the Single-Case Reporting Guidelines in Behavioural Intervention (SCRIBE).

Participants will be recruited via advertisements emailed to schools, shared via social media and word of mouth. After consenting, participants and their parents/carers will complete a baseline eligibility assessment. Participants will complete a range of online questionnaires via a secure online platform. Participants will then attend a remote research assessment appointment to determine whether the family meet the eligibility criteria. The researcher completing the baseline and post-intervention assessments will not be involved in the clinical intervention and will be blinded to baseline allocation. After all baseline assessments for the participant group are completed, participants will be discussed within supervision to confirm that the participants meet eligibility criteria. Eligible participants will then be randomised using a random number generator to one of four different baseline lengths (12 days, 19 days, 26 days or 33 days). The decision to have a minimum of 12 observations per phase meets the requirements for guidelines and standards and will allow for sufficient power for Tau-U analysis.

Each of the participating families will then be contacted to inform them of their allocation. Randomisation and contacting of the families will be completed by a third researcher, who is not involved in either the clinical intervention or research assessments. A letter will also be sent to the participants General Practitioner (GP) informing them of the family's participation in the research study. The setting for the entire study will be remote, with both assessments and intervention sessions being conducted remotely a secure video platform.

All participants in the group will begin their baseline phase concurrently. During the baseline phase participants will be asked to complete the Three-Item Loneliness Scale each day via a secure online platform. SMS message reminders will also be sent to the participants and their parent/carers each day.

The baselines phase will be immediately followed by the intervention. Each family will receive (on average) 12 sessions of CBT for loneliness in children and young people. The first 4 sessions will take place bi-weekly, the second 4 sessions weekly and the final 4 sessions fortnightly.

For all participants the first two sessions will be focussed on assessment and then formulation. The final session for all participants will focus on relapse prevention. During the assessment session the participant will be asked to define 3 goal-based outcomes for how participants hope the intervention will reduce their loneliness, which will then guide the treatment process. During the intervention phase the participants will complete routine outcome measures (ROMs) for each session. This will include the Three-Item Loneliness Scale, visual analogue scales (VAS) of mood, anxiety and loneliness and ratings of their goal-based outcomes. The ROMs will be completed via a secure online platform before each session. If participants have not completed the ROMs before their appointment, then participants will be completed with the clinician at the start of the session.

Participants will begin the post-intervention phase immediately after the final session of their intervention. During the post-intervention phase participants will be asked to complete the primary outcome measure, Three-Item Loneliness Scale each day for 12 days. Participants will then complete their post-intervention assessment with the second researcher. Before the assessment participants will be asked to re-complete the baseline questionnaires in addition to the parent and child-report versions of the Experience of Service Questionnaire and a questionnaire asking whether COVID-19 or any other events have affected their loneliness during the intervention period. A letter will be sent to the participants General Practitioner (GP) informing them of their participation in the research study has finished.