The daily El Periodico de Cataluna reported that the US National Counterterrorism Center (NCTC) alerted Spanish intelligence officers to the threat weeks before a man ploughed his vehicle into crowds of tourists along Barcelona's Las Ramblas boulevard on August 17, killing 14 people.
"Unsubstantiated information of unknown veracity from late May 2017 indicted that the Islamic State of Irak and ash-Sham (IS) was planning to conducted unspecified terrorist attacks during the summer against crowded tourist sites in Barcelona, Spain, specifically (Las Ramblas) street," according to an NCTC briefing note published by the paper.
A spokesman for Spain's CNI intelligence agency, contacted by AFP, refused to "confirm or deny anything on communication with other intelligence services".
The interior ministry did not respond to a request for comment.
But the Madrid-based Cadena SER radio station quoted "anti-terror sources" suggesting that the document published by El Periodico de Cataluna was authentic.
Others were quick to cast doubts on the authenticity of the leaked report, including Wikileaks and its founder Julian Assange.
This appears to be modified or fabricated. The use of Spanish language quote marks '<<', '>>' in an English CIA report is very suspicious.— Julian Assange 🔹 (@JulianAssange) August 31, 2017
And Catalan authorities called a press conference for 12 noon to the report.
El conseller Forn i el Major Trapero compareixen a les 12 hores per desmentir @elperiodico— Mossos (@mossos) August 31, 2017
Spain is still recovering from the twin August vehicle attacks that left a total of 16 people dead and more than 120 wounded in Barcelona and the seaside resort of Cambrils further south.
IS claimed both attacks.