The high pressure fuel injection system utilizes a pressure regulating or control valve (28) in an injector leak-off fuel conduit (26) to provide a high residual pressure within the injector (5) between injections. Maintaining a high residual fuel pressure boosts the injection pressure to provide improved fuel atomization, with consequent reductions in particulate emissions and increased fuel efficiency. The pressure regulating valve (28) may be controlled in accordance with engine operating conditions to provide a variable injection pressure boost commensurate with engine requirements.