It began as an attempt to restore one blurry image that had been hidden for a century behind a large built-in wardrobe on William Morris's bedroom wall.

Months later, the painstaking removal of layers of paint and wallpaper revealed that an entire wall at the artist and craftsman's first married home was painted by his young friends who would become world-famous pre-Raphaelite artists.

The near-lifesize figures on the wall at the Red House, now buried in south-east London suburbia at Bexleyheath, are now believed to represent the joint work of Edward Burne-Jones, Dante Gabriel Rossetti, his wife Elizabeth Siddal, Ford Madox Brown and Morris.

"In the morning we had one and a half murky figures, in the evening we had an entire wall covered in a pre-Raphaelite painting of international importance," James Breslin, property manager at the Red House, said.

"We had no idea what the figures, or the newly revealed inscriptions, represented, but at the Red House it pretty much has to be Chaucer, Arthurian myth or the Bible – all fairly daunting works to start reading line by line."

The property managers decided to tweet an appeal for people to help identify the text, and Breslin said that within an hour a tweet came back saying "Try Genesis 30:6", which reads: "And Rachel said, God hath judged me, and hath also heard my voice, and hath given me a son."

The figures are from the Bible, including Rachel, Noah holding a model ark, Adam and Eve, and Jacob with his ladder – the latter possibly by Morris himself – painted as if on a tapestry furled across the wall.

However the imagery is more complex, because scholars believe it also relates to another cherished pre-Raphaelite Arthurian legend, Sir Degrevaunt who married his mortal enemy's daughter. But then neither family thought much of Morris's choice of Janey Burden, the beautiful daughter of an Oxford stable man.

The mural is also a significant discovery for the National Trust. Ten years ago the National Trust bought the redbrick house studded with romantic details including turrets, stained glass, window seats, a miniature minstrels' gallery and a well, and opened it to the public for the first time.

The property, originally built among Kent orchards, was commissioned by Morris in 1860 from the young left wing architect Philip Webb as a home for his new wife Burden and anticipated large family.

After Morris's day, it passed through a string of private hands and lost almost all the beautiful furnishings designed for it.

Nevertheless it has produced a string of surprises, including traces of wall and ceiling paintings in almost every room. "Basically every white surface in the house is suspect – there will be colour underneath it," Breslin said. "Why have three clashing patterns when you can have six, seems to have been their motto."

In the drawing room, intended by Morris to be "the most beautiful room in the world", panelling was removed to reveal paintings by Burne Jones and probably Rossetti, and some by Morris including roses on a blue background which may be his first attempt at a repeat flower pattern.

Such patterns, manufactured by his company, would by the end of the century cover half the middle-class walls and sofas of England.

In one painting a dog curled up under a chair has turned out to be a wombat, and therefore almost certainly Rossetti's work: he was bizarrely obsessed with the animals, and once owned one as a pet – it died after reputedly eating a box of cigars.

In the bedroom the discovery almost doubled the cost of the conservation work, to £110,000. Fragments showing up on the ceiling and the other walls – partly covered by a particularly horrible 1960s version of Morris's classic willow boughs design, whose owner could never have guessed they were burying a genuine piece by the master – suggest there is much more work to come.

Breslin points out that Morris at Red House was far from "the tub-thumping grey bearded socialist" of later years. They were all young, and it was a party house, devoted to games of hide and seek, music, silly practical jokes and food fights in the drawing room.

Georgie Burne Jones, the artist's wife, recalled that they once stitched the back of Morris's waistcoat so the buttons wouldn't do up, causing him to agonise over growing fat.

The idyll under the apple trees barely lasted five years. Burne Jones's month old son died of scarlet fever, which almost killed his mother, and Siddall died of an overdose of laudanum. Morris, exhausted by the long commute into London, the lovely house half-mortgaged to his mother, sold up. His Guinevere, Janey, went on to have an affair with his best friend Rossetti which almost broke his heart.

"So much of the work was never finished because they were young and they thought they had all the time in the world," Breslin said. "As it turned out, they hadn't."

• The image that originally accompanied this article was replaced on 20 August 2013 as it depicted wall paintings undergoing restoration in another part of the Red House.