Abstract Long COVID has a complex pathology and a heterogeneous symptom profile that impacts quality of life and functional status. Post‐exertional symptom exacerbation (PESE) affects one‐third of people living with long COVID, but the physiological basis of impaired physical function remains poorly understood. Sixty‐eight people (age (mean ± SD): 50 ± 11 years, 46 females (68%)) were screened for severity of PESE and completed two submaximal cardiopulmonary exercise tests separated by 24 h. Work rate was stratified relative to functional status and was set at 10, 20 or 30 W, increasing by 5 W/min for a maximum of 12 min. At the first ventilatory threshold (VT1), was 0.73 ± 0.16 L/min on Day 1 and decreased on Day 2 (0.68 ± 0.16 L/min; P = 0.003). Work rate at VT1 was lower on Day 2 (Day 1 vs. Day 2; 28 ± 13 vs. 24 ± 12 W; P = 0.004). Oxygen pulse on Day 1 at VT1 was 8.2 ± 2.2 mL/beat and was reduced on Day 2 (7.5 ± 1.8 mL/beat; P = 0.002). The partial pressure of end tidal carbon dioxide was reduced on Day 2 (Day 1 vs. Day 2; 38 ± 3.8 vs. 37 ± 3.2 mmHg; P = 0.010). Impaired is indicative of reduced transport and/or utilisation of oxygen. at VT1 was impaired on Day 2, highlighting worsened function in the 24 h after submaximal exercise. The data suggest multiple contributing physiological mechanisms across different systems and further research is needed to investigate these areas.