Objectives: To measure the magnitude of intra fraction prostate motion (IFPM) during stereotactic radiotherapy (SBRT) delivered without intrafraction tracking. To assess if current margins adequately cover IFPM. To derive margins using new guidelines Methods: IFPM was determined in 20 patients receiving 36.25 Gy in 5 fractions using 97 pairs of pre and post treatment cone beam CT (CBCT) scans. Correlation of time between CBCT acquisitions and motion was determined. The magnitude of IFM was compared to current margins (6 mm isotropic, 3 mm posterior). Margins were calculated using IFM alone and updated guidelines. The averaged 3D root mean square IFPM was 2.5 mm (4.2 mm). Independent prostate motion was predominantly posterior (70%) and inferior (63%). There was weak correlation between posterior (ρ = 0.38) and inferior (ρ = 0.36) IFPM and time. IFPM greater than current margins occurred in eight of 97 fractions, six in the posterior direction. Margins were ≤3.5 mm using IFM alone and ≤3.3 mm Left3.5mm Right, 7.0 mm inferior, 3.7 mm superior, 4.4 mm anterior and 3.3 mm posterior using new guidelines, compensating for motion in 92% of fractions. Conclusions: Our current SBRT margins account for 92% of IFPM, predominantly posterior and inferior. Although updated guidelines suggest an increase in margins inferiorly, any increase must be balanced against the possibility of increased toxicity, particularly if biochemical control and side-effects are favourable with current practice. Advances in knowledge: The difference between current clinical margins and those determined using updated guidance is demonstrated. The implications must be considered against clinical outcomes.