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age, long-term condition and residual symptoms of anxiety at end-of- treatment were all significant predictors of relapse. Specifically, as age increases, the risk of a relapse event decreases (hazard ratio = 0.94, p = 0.01). In terms of long-term condition, participants with a long- term condition were over three times more likely (hazard ratios = 3.936, p = 0.02) to experience a relapse event within 9 months. Lastly, participants with residual symptoms of anxiety (GAD-7 score of between 5 and 7) at end-of-treatment were over two times more likely (hazard ratio = 2.94, p = 0.046) to experience a relapse event than those with no or only minimal symptoms. 4 | D I S C U S S I O N This study is the first to explore the remission and relapse rates after an acute phase of treatment with iCBT only, within a routine, stepped- care setting and over a period of 9 months post-treatment. The results add to the scarce literature on the durability of the effects after iCBT treatment in routine care settings, where patients are not typically followed up after receiving a completed course of treatment. Our main findings showed that 70.8% of patients who attained reliable recovery after iCBT treatment remained in remission after 9 months. Younger age, having a comorbid long-term condition, and reporting residual anxiety symptoms at the end of treatment were all indicative of a sig- nificantly higher risk of relapse within the follow-up period. In our study, 29.2% of patients relapsed over a period of 9 months post-treatment. The relapse rates observed here are lower than those reported by Klein et al. (2018) and other studies examining low-intensity interventions including but not limited to iCBT (Ali et al., 2017; Delgadillo et al., 2018). There are several methodological differences across these studies that limit direct comparisons. How- ever, it is important to point out that Klein and colleagues specifically recruited a sample of recurrently depressed participants. Thus, given that it is well established that the risk of recurrence increases as a function of the number of episodes previously experienced (Bruce et al., 2005; Burcusa & Iacono, 2007; Vervliet et al., 2013), it may not be too surprising that their sample of participants had higher rates of relapse. The discrepancies between the relapse rates we observed and those reported by both Ali et al. (2017) and Delgadillo et al. (2018), may possibly be at least partially attributable to the fact that these studies did not distinguish between guided self-help (i.e., bibliotherapy) and iCBT. The literature on relapse rates for biblio- therapy is scarce, but data from two studies with relatively small sam- ple sizes suggest relapse may indeed be higher in this type of therapy (Floyd et al., 2006; Smith et al., 1997). Our study looked exclusively at iCBT interventions that are well-established and embedded in routine care pathways. Additionally, the iCBT programs evaluated in this study continued to be available to participants in an unsupported capacity beyond the end of the treatment period, allowing partici- pants to log in and use the programme for up to a year. This advan- tage that iCBT has over other treatments, including face-to-face therapy, may have played a role in the maintenance of the effects over time. Regarding predictors of relapse at follow up, we found that youn- ger age was associated with higher risk of relapse, which aligns with research on traditional CBT (Gonzales et al., 1985), and some forms of low-intensity CBT (Lorimer et al., 2021). This finding is also consistent with the more general observation that the frequency of episodes of depression and anxiety reduce with age (Asselmann et al., 2019; Blazer, 2010; Jorm, 2000), which in turn may be due to several factors such as age-related reductions in emotional responsiveness, increased emotional control, and psychological immunnisation to stressful events (Blazer, 2010; Jorm, 2000). Residual anxiety symptoms were found to be a predictor of relapse, but these effects were not found for residual depressive symptoms. This was surprising, but given that our definition for relapse stated that relapse could be due to a signifi- cant increase in either depression or anxiety symptoms, it seems that residual symptoms of anxiety played a larger role in determining relapse in our sample. Our results align with previous research in tradi- tional CBT and in low-intensity interventions where residual symp- toms have been consistently identified as predictors of relapse for depression and anxiety disorders (Ali et al., 2017; Lorimer et al., 2021; Wojnarowski et al., 2019). This study also found that the presence of long-term conditions was a significant predictor of relapse at 12 months, which has not been reported in previous research examin- ing predictors of relapse following iCBT, or traditional face-to-face CBT. The absence of evidence for long-term conditions being linked to higher relapse rates to date is likely due to how most studies evalu- ating treatment outcomes for depression and anxiety impose criteria that exclude individuals with long-term conditions in an effort to limit heterogeneity in their study samples. Accordingly, much more research on treatment outcomes and relapse rates for this cohort is warranted. Nonetheless, the increased risk of relapse for individuals with long-term conditions that we observed is compatible with research which has established that comorbid physical and psycholog- ical conditions are associated with poorer patient outcomes than either condition alone (Aquin et al., 2017; Stein et al., 2006). Given that this study was conducted within a naturalistic setting, it offers valuable insights on how services offering iCBT, including but not limited to IAPT, may be able to tailor their care pathways to improve outcomes for patients. For instance, the findings from this study suggest that young people, individuals with residual anxiety symptoms at the end of treatment, and individuals with long-term conditions are more susceptible to relapse following an acute phase of iCBT for depression and anxiety. Services could use this informa- tion to inform them about who might need to be prioritized for the provision of targeted follow-up support which may prevent relapse from occurring. Overall, one could assume that this would help allevi- ate future costs and improve resource allocation within services in the long-term. The effectiveness of psychological interventions for relapse pre- vention has been demonstrated for both depression (Piet & Hougaard, 2011; Vittengl et al., 2007) and anxiety, though for anxiety, this has been limited mostly to panic disorder (van Dis et al., 2020). Despite the evidence base underpinning this, the implementation of relapse prevention programs into routine care services is often 1774 PALACIOS ET AL.