A flow stop valve positionable in a downhole tubular, the flow stop valve comprising: a housing; a sleeve slidably located within the housing; a hollow tubular section slidably located within the housing, the sleeve being provided around and slidably engaged along the hollow tubular section, wherein a piston head is provided at a first end of the hollow tubular section and wherein the hollow tubular section and sleeve together form a valve which selectively permits flow from a first end of the housing to a second end of the housing, wherein the hollow tubular section comprises a port such that the port is selectively blocked by movement of the hollow tubular section or sleeve, the port forming the valve such that in an open position a flow path exists from a first end of the housing, through the port and the centre of the tubular section to a second end of the housing; and a first biasing element; the first biasing element acting on the valve such that the first biasing element biases the valve towards the closed position; wherein the sleeve and hollow tubular section are arranged such that a pressure difference between fluid outside the downhole tubular and fluid inside the downhole tubular acts on the valve, and that the valve is in a closed position when the pressure difference is below a threshold value, thereby preventing flow through the downhole tubular; and the valve is in an open position when the pressure difference is above the threshold value, thereby permitting flow through the downhole tubular; the sleeve and hollow tubular section further being arranged such that when the pressure difference increases by lowering the flow stop valve downhole the first biasing element is preloaded by movement of the sleeve and hollow tubular section together. a housing; a sleeve slidably located within the housing; a hollow tubular section slidably located within the housing, the sleeve being provided around and slidably engaged along the hollow tubular section, wherein a piston head is provided at a first end of the hollow tubular section and wherein the hollow tubular section and sleeve together form a valve which selectively permits flow from a first end of the housing to a second end of the housing, wherein the hollow tubular section comprises a port such that the port is selectively blocked by movement of the hollow tubular section or sleeve, the port forming the valve such that in an open position a flow path exists from a first end of the housing, through the port and the centre of the tubular section to a second end of the housing; and a first biasing element; the first biasing element acting on the valve such that the first biasing element biases the valve towards the closed position; wherein the sleeve and hollow tubular section are arranged such that a pressure difference between fluid outside the downhole tubular and fluid inside the downhole tubular acts on the valve, and that the valve is in a closed position when the pressure difference is below a threshold value, thereby preventing flow through the downhole tubular; and the valve is in an open position when the pressure difference is above the threshold value, thereby permitting flow through the downhole tubular; the sleeve and hollow tubular section further being arranged such that when the pressure difference increases by lowering the flow stop valve downhole the first biasing element is preloaded by movement of the sleeve and hollow tubular section together.