The UDB was founded in 1964 in Rennes by a group of about fifteen young people, most of them students, often from cultural organizations (Ar Falz, Bagadou, etc.)

The majority belonged to the "Mouvement pour l'organisation de la Bretagne" organization which also included former militants of Breton Nationalist Party and supporters of French Algeria, which was a cause of tensions.

Internal contradictions within the party exploded during the 1969 congress when Alain Guyader challenged the charter of the UDB and proposed a line inspired by Rosa Luxemburg's ideas.

The conflict led to the exclusion of Jean-Yves Guiomar and Alain Guyader in 1970 for "rejection of democratic centralism, constant undervaluation of the adversary, impatience and the theory of active minorities, and the idolization of spontaneity".

It also adopted a Marxist line and demanded in a modified version of its charter the abolition of capitalism and the collective appropriation of the means of production.

At the same time, a UDB list won 11.5% of the votes in a local by-election in Auray, primarily thanks to the personality of its candidate Sten Kidna.

In the 1971 local elections the UDB took part in the lists of the Union of the Left, except in Brest where it polled 4.8% running independently.

Party growth slowed as a result of failure of the legislative elections and the collapse of the United Left, in which the UDB was firmly anchored.

In 1984, during the Lorient congress, one of the sections from Léon, whose motion had won a third of the votes, created a splinter group, Frankiz Breizh, based primarily in Brest and its immediate surroundings.

In the 2001 local elections it chose to take part in lists presented by the united left except in Guingamp, Redon, Lannion and Saint-Nazaire.

A separate regionalist list could have won between three and five percent, thus cutting into the vote share of the ecologists and condemning their efforts to assert their independence from the Socialist Party.

In the 2009 European Parliament election, the party supported the Europe Écologie electoral coalition, which included The Greens.

[5] He stood for the UDB as a Europe Ecology – The Greens candidate in Morbihan uncontested by their electoral allies, the Socialist Party.

[7] During the 2017 legislative elections, the UDB, together with the MBP, reactivated the dynamics of "Oui la Bretagne" (Yes Brittany) by aligning 34 candidates on 37 constituencies.