Four decades after the end of the Franco dictatorship, the wall surrounding Spain’s largest mass grave is finally to come down.

After a six-year battle, the family of two brothers executed by General Francisco Franco's troops in 1936 will on Monday see work begin on exhuming their remains from the Valley of the Fallen, the controversial monument containing Franco's tomb where at least 33,000 Civil War victims are also interred.

A team of archaeological and forensic specialists will break through the wall surrounding the monument’s vast ossuary in search of the bodies of Manuel Lapeña, a leftist union leader and father of four, and his metalworker brother Antonio.

It will mark a pivotal moment in the long fight by victims’ families to finally locate their loved ones, whose fates have mostly been obscured beneath amnesty laws imposed after the death of Franco.