A hospital has denied it needs to spend £65m to replace faulty plumbing in its 21-year-old building.
Copper pipes showed problems at Ysbyty Gwynedd after 10 years
Ysbyty Gwynedd said problems with its copper pipes started to show within 10 years of the Bangor hospital opening.
But it said replacing the pipework would cost £20m, not the £65m quoted in a note disclosed by the assembly under Freedom of Information (FOI) laws.
North West Wales NHS Trust said the pipes bill was part of extensive refurbishment plans costed at £54m.
The figure of £65m is suggested in a briefing document on capital investment compiled by the Welsh Assembly Government and released under FOI regulations.
It reads: "The corrosion of copper water services at Ysbyty Gwynedd was first cited as a potential problem in 1993 and has been closely monitored and maintenance carried out.
The new proposals would improve facilities as well as the plumbing
"In order to avoid possible major failures and the consequences, it is proposed that all copper pipe-work services within Ysbyty Gwynedd should be replaced at an estimated cost around £65m."
But the figure is a combination of the overall refurbishment plans for both Ysbyty Gwynedd in Bangor and Wrexham Maelor Hospital in Wrexham.
North West Wales NHS Trust, said its current refurbishment estimate of £54m included the pipes' work as well as plans to "substantially upgrade and modernise hospital facilities".
Wrexham Maelor Hospital has bid to spend £10m on refurbishment, including £1m on new plumbing, with a further £5m on a temporary working space for wards while any upgrade is carried out.
In a statement, North West Wales NHS Trust said: "Ysbyty Gwynedd was opened in 1984.
"Due to a combination of factors, the pipes have been prone to leaking and this has posed a risk of catastrophic loss of service and general health and safety hazards.
"The work in involved with the replacement of copper pipes throughout the hospital will cause significant disruption."
The trust said the new proposals would expand the hospital's high dependency unit from two beds to six, improve facilities for teenagers on the children's ward and bring extra toilets and shower facilities.
They would also involve installing double glazing, better insulation and energy-efficient lighting, as well replacing the existing pharmacy, it said.
'No final decision'
An assembly government spokesperson said no decision had been made on what repair options, which started at £20m, would be pursued at the hospital
An assembly government spokesperson said: "The cost of options for carrying out the work, which are being considered at the moment, vary from £20m to £54m.
"We are at an early stage of the business case process and no final decisions have been taken on which option will be chosen.
"The work on the copper-piping has to be carried out in order to protect the rest of the building."