A law granting amnesty to those prosecuted over Catalonia's attempt to secede from Spain was included in the deal, according to a joint PSOE-Junts statement seen by Reuters.

Santos Cerdan, a senior Socialist party official who has led negotiations, told a press conference in Brussels that while his party still had "profound disagreements" with Junts, it had put them aside in the interests of forming a "stable government", and the agreement included support for a full four-year term.

Among the beneficiaries of an amnesty is Carles Puigdemont, the Junts leader currently living in exile in Waterloo, Belgium, because of charges he himself faces as leader of Catalonia during the separatist drive that came to a head in a vote and unilateral declaration of independence in 2017.

If the amnesty is approved by congress, Puigdemont would be able to return to Spain and potentially run for office.

Puigdemont hailed the agreement between Junts and the Socialists as a "change of narrative" and a step towards resolving a "historic conflict" between Spain and the wealthy eastern region.

Puigdemont told a press conference in Brussels that the amnesty offered reparation for what he described as "political persecution" by Madrid, and a guarantee it would not be repeated.

He said Catalans neither had to recognise they had committed any crimes as they agitated for independence in mass demonstrations, votes and a unilateral independence declaration, nor apologise for what happened - something the Socialists had originally insisted on in negotiations.

Sanchez is trying to form a government after a July election produced no outright winner.

He reached a deal to govern in coalition with the hard left Sumar platform last month but also needs several other smaller parties - who have supported him in the past - to back him in an investiture vote that could take place as soon as next week.

At present, even with Junts' support, the Socialists would still fall short of an absolute majority of 176 seats to win a first round vote in the 350-seat congress, or to secure a simple majority in a second vote.

They still need the support of five lawmakers from the Basque Nationalist Party (PNV), which offered early support but said it would wait to see the details of a Socialist-Junts deal before confirming.

The vote has to be concluded by Nov. 27 or fresh elections are automatically triggered.

An amnesty could exculpate as many as 1,400 activists and politicians involved in the attempt to separate Catalonia from Spain.

It has met fierce condemnation from Sanchez's conservative opponents who have organised large protests and accused him of putting the rule of law in Spain on the line for his own political gain.

Reacting to the news of a deal on Thursday, the main opposition centre-right People's Party spokesman Cuca Gamarra said Spain was looking at "a shameful and humiliating deal."