Plans by Irish private equity group Ion to build an 18-storey Pillo hotel in Manchester's Deansgate Locks area were criticised by CABE, the design watchdog.
CABE's design review panel urged the council's planners, who recently granted planning consent for the scheme at 10-12 Whitworth Street West, to demand more reassurance on the materials to be used on the outside of the new building given its historic location.
CABE said: "The current application leaves much open to interpretation, and as a result, there is scope for significant reduction in quality.
"The opportunity to influence the quality of the facades will be limited once an approval is in place. Therefore, in view of our concern about the credibility of the design, we urge the local authority to request that much more detailed and prescriptive information on the proposed cladding systems and key construction details is added to the application before it is determined, rather than allowing this to be dealt with by condition.
CABE concluded: "We cannot lend our support to this application without evidence that the approach to materials and detailing has been more definitively secured."
The hotel was designed by HKR Architects and will contain 285 rooms. The existing building on the site, sold in January by Ask Developments, will be demolished to make way for the hotel.