A pressure vessel includes front and rear end plates and a plurality of open-ended vessel structures constructed of fibre-reinforced polymer matrix composite material positioned adjacent to one another so that their longitudinal axes are parallel to a longitudinal direction extending between the front and rear end plates. The vessel also includes an outer reinforcement comprising polymer matrix composite material with continuous fibres extending longitudinally around the pressure vessel to secure the front and rear end plates to the vessel structures. At least one of the vessel structures has a partially curved cross section in a plane perpendicular to its longitudinal axis, such that one or more crevices are formed between the vessel structures, running longitudinally between the front and rear end plates and the front and rear end plates are shaped to allow the outer reinforcement to at least partially fill the one or more crevices between the vessel structures.