Once in Britain, Petroff contacted the Social Democratic Federation, a socialist organisation sympathetic to the Russian Marxists. They put him in touch with John Maclean, who for two months housed him in Glasgow and taught him English. Petroff then moved to London and began working for the German Social Democratic Party. He also contributed to La Voix du Social Democrat and Russkoe Slovo. As Petroff's English improved, he began speaking on behalf of the SDF, and writing regularly for party newspapers. However, he became a leading opponent of the party's leadership, which he felt was ineffective, undemocratic, and nationalistic. The SDF reformed as the British Socialist Party in 1912, and Petroff was elected to its first standing orders committee, alongside Duncan Carmichael, E. C. Fairchild and C. T. Douthwaite. The four worked together to ensure voices in the party opposing British rearmament were heard. Maclean shared Petroff's views on the party leadership, and led an unsuccessful campaign in 1914 for a reduction in the leadership's control and also for a more stable party programme, adopting one overall programme for each general election. Petroff stood as an anti-militarist candidate for the executive of the BSP that year, but was defeated by party leader H. M. Hyndman. Instead, he accepted work as the political organiser of the Glasgow branch of the BSP.
Red Clydeside
An opponent of World War I, he gave talks which attracted large crowds and influenced members of the Clyde Workers Committee. He criticised the CWC for focusing solely on industrial action and not campaigning on wider political issues. Nonetheless, he wrote articles on the progress of the movement for Nashe Slovo and Berner Tagwacht, raising its profile among socialist anti-war activists across Europe. Increasingly alarmed by the growth of anti-war feeling in the party, Hyndman attacked his opponents through the party's newspaper, Justice. A December 1915 article, "Who and What is Peter Petroff", gave useful information to the authorities about Petroff's activities, and within the next month he was twice fined for breaking the Aliens Protection Order, then imprisoned for two months. In March, instead of being released, he was interned, firstly at Edinburgh Castle, then in Islington; his wife, Irma Gellrich, was interned separately.
In January 1918, Petroff and Gellrich were repatriated to the Soviet Union alongside Georgy Chicherin, the British government acceding to a request by Trotsky; Petroff was made Vice-Commissar for Foreign Affairs, taking over from Zalkind. He subsequently served as Chairman of the Foreign Relations Committee, and Chairman of the Political Section of the Supreme Military Inspection of the Red Army. Although barred from visiting the UK, he remained in contact with Maclean, and also Tom Quelch and James Clunie. In 1921, he was sent to Germany, to support the communist party there. He was sympathetic to Trotsky's Left Opposition, and resigned from the Bolshevik Party in 1925, but remained active in the German communist movement until the Nazi rise to power. Petroff and Gellrich fled to Britain in 1933, where he worked as a journalist, principally for overseas newspapers. Given his opposition to Stalin, he faced hostility from the Communist Party of Great Britain, and instead joined the Labour Party, writing extensively for its journal, Labour. In 1934 he and his wife wrote The Secret of Hitler`s Victory: The Causes of the Breakdown of the German Republic published in hardback by the Hogarth Press. With the onset of World War II, his work dried up, but he remained in London until his death, eight years later.
