Background Atrial fibrillation (AF) is a major cause of stroke and heart failure, often remaining undiagnosed when asymptomatic. Patients with Type 2 diabetes mellitus (T2DM) are at increased risk due to metabolic and structural cardiac alterations, yet the true prevalence of silent AF within this group remains uncertain. This study aimed to determine the prevalence of silent AF using extended Holter monitoring and to identify clinical and echocardiographic predictors among T2DM patients in the United Kingdom. Methods This prospective multicentre cohort study was conducted across three NHS hospitals between January 2023 and June 2025. A total of 420 adults with T2DM (mean age 62.4 ± 8.7 years; 54.8% male) and no prior AF were monitored using 7-day continuous Holter ECG. Baseline clinical, biochemical, and echocardiographic parameters were collected. Silent AF was defined as ≥30 s of irregularly irregular rhythm without symptoms. Multivariable logistic regression was used to identify independent predictors. Results Silent AF was detected in 46 participants (10.9%) [95% CI 7.9–14.2]. Compared with those without AF, affected individuals were older (66.1 ± 7.9 vs 61.9 ± 8.6 years, p = 0.002), had longer diabetes duration (12.7 ± 5.4 vs 9.6 ± 4.8 years, p < 0.001), higher HbA1c (8.3 ± 1.1% vs 7.8 ± 1.2%, p = 0.004), and greater left-atrial volume index (LAVI) (38.7 ± 6.9 vs 32.4 ± 5.8 mL/m², p < 0.001). Independent predictors of silent AF included age ≥65 years (aOR 2.04, 95% CI 1.05–3.95), diabetes duration ≥10 years (aOR 2.56, 95% CI 1.34–4.89), HbA1c ≥7.5% (aOR 1.98, 95% CI 1.03–3.79), LAVI >34 mL/m² (aOR 3.21, 95% CI 1.65–6.27), and neuropathy (aOR 2.44, 95% CI 1.16–5.13). The model’s discrimination was excellent (AUC = 0.82, p < 0.001). Conclusions Approximately one in ten T2DM patients in the UK harbours unrecognised atrial fibrillation detectable by 7-day Holter monitoring. Prolonged ECG surveillance should be considered in diabetics aged ≥65 years or with poor glycaemic control, structural atrial changes, or neuropathy. Early identification of silent AF in this high-risk population may enable timely initiation of anticoagulation and significantly reduce preventable stroke burden within NHS care pathways.