British warships no longer targets for Soviet submarines according to the Weekly Intelligence Summary No. 19 for the week ending19 March 1921

An item reported that Soviet submarines no longer considered British warships to be hostile and just no longer to be attacked. The British ships were however still ordered to take all needed precautions. The Soviet government was asked that her submarines only with daylight, surfaced with their colours hoisted would approach British warships.

Source

The National Archives, Kew Gardens, England CAB-24-121-62