Little is known about the existence of extrasolar planets around ultracool dwarfs. Furthermore, binary stars with Sun-like primaries and very low-mass binaries composed of ultracool dwarfs show differences in the distributions of mass ratio and orbital separation that can be indicative of distinct formation mechanisms. Using FORS2/VLT optical imaging for high precision astrometry we are searching for planets and substellar objects around ultracool dwarfs to investigate their multiplicity properties for very low companion masses. Here we report astrometric measurements with an accuracy of one tenth of a milli-arcsecond over two years that reveal orbital motion of the L1.5 dwarf DENIS-P J082303.1-491201 (having 7.5+/-1% of the Sun's mass) caused by a companion with a mass of 28+/-2 Jupiter masses that revolves about its host on an eccentric orbit in 246.4+/-1.4days. This new system is nearby at 20.77+/-0.08pc and has the smallest mass ratio (0.36+/-0.02) of known very low-mass binaries with a characterised orbit. With this discovery we demonstrate 100 micro-arcsecond astrometry over an arc-minute field and over several years that is sufficient to discover sub-Jupiter mass planets around ultracool dwarfs. We also show that the achieved parallax accuracy of <0.4% makes it possible to remove distance as a dominant source of uncertainty in the modelling of ultracool dwarfs.