Picasso’s cold war career as a highly political painter, peace campaigner and tireless fundraiser for leftwing causes will be revealed in an exhibition at Tate Liverpool next spring that will include letters from world leaders, including Nelson Mandela and Ho Chi Minh, as well as a telegram from Fidel Castro congratulating the artist on being awarded the Soviet Union’s international peace prize.

Christoph Grunenberg, the gallery’s director, said the exhibition would explode the myth that Picasso was “a playboy extrovert … more concerned with chasing women than world politics”.

Picasso himself said: “I have not painted the war because I am not the kind of painter who goes out like a photographer for something to depict. But I have no doubt war is in these paintings.”

The exhibition begins in 1944, the year he joined the French communist party. He remained a member until his death in 1973, and Lynda Morris, the curator, said the legend that he was the party’s largest individual donor is probably true.

He rarely gave money, but gave innumerable works to be reproduced as fund raising calendars, Christmas cards, silk scarves or limited edition prints, so many that the Communist journal l’Humanité had a full time staff member working with him on producing them.

She found dozens of boxes of political correspondence in the archives of the Picasso Museum in Paris, showing that he was in constant touch with peace groups, refugee aid schemes and women’s groups, in Europe, north and south America, and Israel. He also supported hospitals and homes in France sheltering refugees from the Spanish civil war.

The exhibition opens with a painting last seen in Britain half a century ago, the 1944 Charnel House, with echoes of his famous Guernica, inspired by the first horrific images from the liberated concentration camps, and newspaper accounts of a Spanish Republic family killed while sheltering in their kitchen.

It will include several versions of his dove drawings, originally modelled on the fan tailed pigeons given him as a present by the painter Henri Matisse.

His doves became symbols recognised across the world of the peace movement, after one was chosen as the emblem of the first international peace congress in Paris in 1949 – the same month he named his daughter Paloma, the Spanish for dove. He produced new versions of the design for posters for each of the later peace congresses including the Sheffield gathering, planned at the height of the Korean war, when Picasso himself was held by immigration for several hours, and which was abandoned after the Labour government of the day refused entry to hundreds of delegates including the American singer Paul Robeson, and the writers Pablo Neruda, and Louis Aragon.

The exhibition will not be seen in London, and builds on the success of the Liverpool gallery’s success with its major Gustav Klimt show, one of the hits of last year’s European Capital of Culture. Lynda Morris said it never occurred to her to approach a gallery in the south – the radical tradition of the north made it the right place for the show.