They have survived the religious purges of the Reformation, the sectarian violence of the English civil wars, even the widespread whitewashing of church walls in the 19th century.

But now hundreds of priceless medieval panel paintings in churches across East Anglia – depicting gruesome scenes from hell, biblical figures, English monarchs and angels and apostles – face a new and potentially deadly threat: deathwatch beetles.

Infestations of insects and bats are among the problems increasingly bedevilling Britain’s ancient, leaky churches which are struggling to pay for repairs as congregations and income dwindle. Deathwatch beetles are boring through wood, creating holes twice the size of those made by woodworm; bats are urinating over paintings, causing white spotting and chemically altering the paint; while moisture and fluctuations in humidity are also causing paint to flake.

Various bodies, including the Hamilton Kerr Institute at Cambridge University and the Church Buildings Council, are involved with a major project to save these treasures. They have submitted a bid for about £1m to the Arts & Humanities Research Council, warning of the need to investigate, safeguard and record these treasures “before they are lost due to theft, neglect or lack of awareness”. A decision is expected shortly.

Their submitted bid notes: “East Anglia is extremely rich in late-medieval painted wooden church screens. Examples are found in more than 40% of the medieval churches, putting the lie to the received wisdom that all pre-Reformation art was violently smashed.”

Most are rood screens. Sited between the nave and chancel and formerly topped by a crucifix (rood), more than 500 examples survive – many stripped of their paint – within the historical dioceses of Norwich and Ely, “in higher density than anywhere else”.

Around 200 of 350 medieval paintings in Britain are in East Anglia. The report notes: “These objects are extremely vulnerable… Screens are threatened by humidity, temperature, light and degradation from pests. Without a conservation strategy, they will be lost for ever.”

Dr Lucy Wrapson, a Hamilton Kerr senior conservator, said “there’s really no question” that paintings will be lost: “You lose paint over time, and once you lose it, it’s irreplaceable. It’s a slow drip, drip, drip. Unless you’re able to keep the building intact and drain the rainwater, they can be terribly vulnerable to damp and environmental problems.”

While there are “very good” examples of medieval paintings in Devon, Cornwall and Somerset, that there’s nothing comparable to East Anglia’s collection, Wrapson said: “Barton Turf and Ranworth are among the most exquisite screens in Norfolk, and Bramfield in Suffolk is also a gem. They are all beautifully painted in bright oil colours using pigments such as vermillion. The lower parts of these screens have the painted figures of saints and angels along the dado.”

At Ranworth, images include dramatic figures of St Michael and St George slaying their respective dragons. At Barton Turf, they feature St Apollonia, patron saint of dentistry, holding a tooth in her pincers. Astonishingly, despite their importance, these paintings have yet to be studied properly in their entirety. Wrapson and Paul Binski, professor of medieval art history at Cambridge University, are planning the first corpus of British medieval panel painting.

Wrapson said: “Nobody realises how much of this stuff there is out there. It’s not collated in one place, so a lot of scholarship doesn’t happen. We really want to know what’s there.”

She has, for example, discovered a portrait of Henry VII on the North Tuddenham rood screen. St Sebastian has been given the “pinched face” features of the king, who was on the throne at the time of its making.

The project involves Tobit Curteis, a leading architectural conservator, who said: “There are some stunners out there. You walk into these little isolated churches and there is this absolutely remarkable painting sitting there... We have this fantastically important collection of artefacts and the wider public doesn’t know they are out there. These paintings of angels, saints, dragons would have been an absolutely core part of the colour, mystery and magic of the medieval church. They’re sitting in these churches, often with a congregation of a dozen. They are incredibly sensitive and vulnerable, the sort of paintings that would normally be hanging in museums. Yet here they are, in some of our oldest and most difficult to look after churches. They are desperately in need of care and conservation. Otherwise they will deteriorate and disappear, which would be a complete tragedy.”

Part of the project is to make people more aware of their significance. Wrapson despairs at past decisions to hammer nails into screens, poke wires through them and even spray silver paint over Christmas decorations draped on them.

Asked why the paintings are now at risk, Curteis said: “With the declining church population, it is harder to raise money to look after these buildings.”

Beyond leaking roofs and water coming up through the walls, he said that deathwatch beetles tend to colonise moist wood: “They eat their way through the structure when they’re hatching. In some cases, they eat so much it becomes structurally unstable and it collapses. When you see a panel which has been attacked by deathwatch beetle, it’s peppered with holes like a shotgun blast. If you can control the moisture in the timber, the beetles choose not to live there.

“Bats urinate and defecate as they fly. The material they produce is extremely caustic. It burns into the paint layer.” Attempts are being made to combat bats with measures like false ceilings.

He added that these paintings have survived 500-600 years: “If they deteriorate over 25 years, that is phenomenally fast... Will we lose some of these artworks before our grandchildren are around to see them? Yes. There is an opportunity now to take action – but that takes a lot of time and effort.”