Atmospheric gravity waves with very long crests (of 450-500 km length) and short horizontal wavelengths of about 20 km were observed in noctilucent clouds and were studied in detail for the first time. The gravity waves were slowly moving in opposite direction to the background wind indicating their forced generation outside the mesopause region. A ray-tracing analysis using meteorological reanalysis and empirical atmospheric model data shows that a source of such peculiar gravity waves observed in noctilucent clouds was located near the tropopause and could be associated with the jet stream at altitudes 8-10 km. Two considered examples of very long wave crests confirm a significant role of the upper tropospheric jet stream as a source of gravity waves and reveal that these waves could propagate without critical levels to the mesopause in summertime.