An overspend of £9.3m on the redevelopment of the Royal Albert Memorial Museum (RAMM) in Exeter has been criticised by a city councillor.

Liberal Democrat Kevin Mitchell told local press that “the public deserves to know why the council is borrowing money to ensure that this vital project is completed”.

The original budget for the RAMM’s refurbishment was £15m. The museum is scheduled to reopen in December, but was originally due to open last year. Exeter City Council has been the main funder of the revamp.

The Heritage Lottery Fund has allocated £10m, while the DCMS/Wolfson Fund and Garfield Weston Foundation have provided smaller awards.

A museum spokesman said the overspend was mainly the result of delays following the discovery that the Norman defence ditch of Exeter Castle, over which the rear part of the museum was built in the 1880s, is considerably deeper than original surveys showed.

“This led to substantial redesign and lengthy safety works to protect the building, which was sinking into the ditch,” he added.

But Mitchell said “a large proportion of the overspend is due to other factors which arguably can be attributed to actions and decisions made at the beginning of the project, and these are still not in the public domain”.

A council spokesman said the complications caused by the ditch were the main reason for the budget rise. “Until the building project comes to an end, the council cannot air the issues in public, as this could prejudice its contractual position,” he added.

The redevelopment involves building a gallery parallel to the city’s Roman wall that will provide more space for temporary exhibitions.