An urgent £250,000 upgrade is needed for the sewerage system at Norfolk's rural life museum after a series of leakages were reported from storage tanks.

The foul sewage treatment plant at Gressenhall Farm and Workhouse was installed in 2001 and designed for average annual attendances of about 40,000.

But with visitor numbers now averaging 75,000 per year – and projected to rise to 80,000 or 90,000 during the next five years – councillors have been told that the system is no longer able to cope.

A report to Norfolk County Council's cabinet says: 'In the last five years, a number of serious incidents have occurred where raw and untreated sewage has overflowed from the existing storage chambers and seeped into the surrounding areas of woodland.

'There has been no contamination of the local river system up to this point but there is a high risk that contamination may occur. Contamination of the river system would incur significant legal, financial and reputational risk to the council.'

Robin Hanley, western area manager for the Norfolk Museums and Archaeology Service, said: 'We take environmental concerns very seriously here, so any sewage emerging from the system is a great concern to us.

'That is why the wording in the report is so stark, but what we are talking about is something that's very localised.

'It was a small amount of seepage in the immediate area of the tanks, in an area of woodland that is not accessible to visitors.

'It absolutely has not gone into any water courses. But we take any leakage extremely seriously, and that is why we have flagged this up with the council members. We have great aspirations to grow the visitor numbers here at Gressenhall and with that in mind we are very aware that the site infrastructure needs to be fit for purpose.'

The report outlines proposals costing £253,125 which it says would accommodate projected visitor growth, and comply with the strict requirements set down by the Environment Agency.

When cabinet members meet on Monday, they will be asked to agree to the improvements being taken forward 'subject to funding for the on-going revenue costs of borrowing the money being identified'.