The Basque militant group Eta says it will hand over all its weapons on Saturday - but warns that its enemies might still block the process.
The pledge came in a letter obtained by the BBC, confirming earlier reports about Eta disarmament plans.
The Spanish government said Eta "will get nothing" in exchange. It demanded that Eta "disarm and dissolve itself".
Eta killed more than 800 people and wounded thousands in over 40 years of violence aimed at Basque independence.
The group declared a ceasefire in 2011 but did not disarm. Spain refuses to negotiate with them.
Speaking on Friday, Spanish government spokesman Inigo Mendez de Vigo said Eta "will get nothing from a democratic state like Spain".
He demanded that the group not only disarm, but also clarify who carried out past Eta attacks.
The Eta letter, dated 7 April, says that "after giving up all its weaponry (arms and explosives) to Basque civil society representatives [Eta] now is a disarmed organisation".
But further on it warns that "the process is not completed" and "'disarmament day' is tomorrow".
"We want to warn that still the process can be attacked by the enemies of peace. The only real guarantee to succeed are the thousands of people gathering tomorrow in Bayonne [south-western France] supporting the disarmament."
In recent years police in France and Spain have put Eta under severe pressure, arresting hundreds of militants, including leadership figures, and seizing many of the group's weapons.
'A bold move' - the BBC's Guy Hedgecoe in Madrid writes:
When Eta announced the end of its campaign of violence, in October 2011, disarmament was the next logical development. The fact it has taken the organisation so long to take that step probably reflects the reluctance of many within its ranks to do so when the Spanish and French governments have offered nothing in exchange.
Eta's assertion that the "process is not completed" highlights a desire to see concessions in the future, particularly the transfer of many of almost 400 jailed members to prisons nearer their families.
But in the apparent absence of any such promises from Madrid or Paris, decommissioning is a bold move. It paves the way to Eta's eventual disbanding and, possibly, to some form of reconciliation in Basque society.
A key figure in Eta's move away from violence is believed to be the leader of the pro-independence Basque left, Arnaldo Otegi, who has served several jail terms for his links to the organisation. However, some Eta prisoners criticised him last year for compromising too much on their behalf.
Bayonne, in France's Basque region, is expected to be the focus of the disarmament process on Saturday.
Spanish media have speculated that Eta may not have many weapons left, after so many successful police operations.
Last month France's Le Monde daily reported that a pro-Basque independence and environmental group called Bizi had been given responsibility for the disarmament. It was quoting a Bizi activist.
Eta was set up more than 50 years ago in the era of Spanish dictator General Franco. Its first known killing was in 1968, when a secret police chief, Meliton Manzanas, was shot dead in the Basque city of San Sebastian.
In 2014 the International Verification Commission of international inspectors said Eta had put some of its weapons out of action, but the Spanish government dismissed the move as "theatrical".
Spain's El Pais newspaper says the key moment, if Eta disarmament goes ahead, will be when the International Verification Commission notifies France about the locations of Eta arms caches. The commission is led by Ram Manikkalingam.
The Basque regional government - which has a large degree of autonomy under Spanish law - says those locations will remain secret.
Basque government sources quoted by El Pais said they would like Eta to formally dissolve itself, rather than wait for the group to fall apart gradually.
By formally disbanding, they argued, it would be easier to persuade the Spanish authorities to transfer jailed Eta militants to prisons closer to their Basque families. Dissolution of Eta might also ease their prison conditions, they said.
A historic moment: By the BBC's Chief International Correspondent Lyse Doucet
Sources say Eta has tried for years to start secret talks with the Spanish and French governments. On at least three occasions, an opening with French officials was blocked by Spain.
It's understood that in the last month, once Eta's plans to disarm unilaterally were leaked, French assurances were received that a ceremony to hand over coordinates for its weapons caches could take place in France.
There have also been expressions of support from Spanish parties across the political spectrum, but not the Popular Party of Prime Minister Mariano Rajoy.
Large numbers of people from the Basque regions of Spain and France are expected to attend a day of events on 8 April. Critics dismiss the disarmament as theatre.
For some it is mainly symbolic, since much of Eta's arsenal is believed to be obsolete. But for many, it will be an historic moment which marks the end of the last insurgency in Europe.