Unesco has warned that Liverpool's world heritage waterfront would be "irreversibly damaged" if Peel Group's £5.5bn Liverpool Waters scheme goes ahead as planned.
A report, which follows a visit by inspectors to the city in November, says it will not support Liverpool Waters in its current form and has called on the council, Peel and English Heritage to work out an alternative.
Peel wants to build a series of towers in an office-led development on 150-acres of the city's north docks.
The report says the historic north docklands complement those to the south, putting the Three Graces centre-stage in, more or less, a symmetrical profile.
"This has a historical reason as the Three Graces were at the heart of shipping and harbour operations during the height of its glory.
"Shifting this profile to the north by introducing a secondary cluster of high-rises, with towers three times the height of the Three Graces, would destroy this profile, relegating the Three Graces to playing second violin, and thereby losing an important visual and historical reference to the city's glorious past."
It adds: "We conclude if the proposed Liverpool Waters scheme, as outlined, would be implemented, the World Heritage property would be irreversibly damaged, due to a serious deterioration of its architectural and town-planning coherence, a serious loss of historical authenticity, and an important loss of cultural significance."
Cllr Joe Anderson, leader of Liverpool City Council, said: "I welcome the findings of the World Heritage report and it's really encouraging they are pressing for a compromise which would enable Liverpool Waters to go ahead.
"I have always believed there is a way forward which will allow us to redevelop the North Liverpool Docks and secure the massive investment and badly-needed new jobs, and to also preserve our World Heritage status. Peel have already made significant alterations to their proposals since drawing up the original plans."
He added: "Unesco's delegation found the conservation of our World Heritage site has improved since their last visit in 2006 with our new developments at the Pier Head, at Liverpool One and in the Ropewalks area. Our track record clearly demonstrates we can be trusted to combine the old with the new.
"I am pressing English Heritage, Peel and the city council to redouble their efforts to reach agreement on the best way forward for the Liverpool Waters plans."
Peel has not yet issued a statement on Unesco's findings.
Read the full report here.