This paper is part of a series discussing the results obtained in the framework of a wide international collaboration - the Lockman Hole Project - aimed at improving the extensive multiband coverage available in the Lockman Hole region, through novel deep, wide-area, multifrequency (60, 150, 350MHz, and 1.4GHz) radio surveys. This multifrequency, multiband information will be exploited to get a comprehensive view of star formation and active galactic nucleus activities in the high-redshift Universe from a radio perspective. In this paper, we present novel 1.4GHz mosaic observations obtained with the Westerbork Synthesis Radio Telescope. With an area coverage of 6.6deg^2^, this is the largest survey reaching an rms noise of 11{mu}Jy/beam. In this paper, we present the source catalogue (~6000 sources with flux densities S>~55{mu}Jy (5{sigma}), and we discuss the 1.4GHz source counts derived from it. Our source counts provide very robust statistics in the flux range 0.1<S<1mJy, and are in excellent agreement with other robust determinations obtained at lower and higher flux densities. A clear excess is found with respect to the counts predicted by the semi-empirical radio sky simulations developed in the framework of the Square Kilometre Array Simulated Skies project. A preliminary analysis of the identified (and classified) sources suggests this excess is to be ascribed to star-forming galaxies, which seem to show a steeper evolution than predicted.