Anxiety and depression often co-exist. These disorders are under-diagnosed and under-treated, specifically among older people, and lead to increased use of health and social care services and raised mortality. Older people report a reluctance to present to their GP with depression or anxiety symptoms due to perceived stigma about mental health problems, lack of acceptable treatments and the prioritising of physical health problems. Third sector organisations, who work closely with older people in the community, are well-placed to provide additional support. We developed a brief intervention based on principles of Behavioural Activation, with encouragement to participate in a group activity, for delivery by Support Workers from AgeUK. The aim of the study was to examine whether this brief intervention could be delivered to older people with anxiety and/or depression, with sufficient fidelity, and whether this approach was acceptable to patients, GPs and AgeUK Support Workers. Semi-structured interviews with older people with self-reported anxiety and/or depression (who received the intervention), Support Workers and GPs to assess acceptability of the intervention and impact on routine care. A constant comparative approach was used to analyse the data. Intervention sessions between Support Workers and older people were digitally recorded and reviewed by the research team to assess fidelity. The Support Workers delivered the intervention with fidelity; access to the training maual and ongoing supervision were important. Older people found the intervention acceptable and valued the one-to-one support they received; group activities suggested by Support Workers were not valued by all. GPs recognised the need for additional support for vulnerable older people, but acknowledged they could not provide this support. Participation in the study did not impact on GP routine care, other than responding to the calls from the study team about risk of self-harm. Support Workers within AgeUK, can be recruited and trained to deliver an intervention, based on the principles of Behavioural Activation, to older people with anxiety and/or depression. The training and supervision model used in the study was acceptable to Support Workers, and the intervention was acceptable to older people and GPs. This model has the potential to contribute to improving the support and care of older people in primary care with anxiety and depression. Further testing is required in a full trial. Trial registration number ISRCTN16318986 . Registered 10/11/2016.