Damage done to a Mark Rothko painting when it was defaced at the Tate Modern gallery in October is much worse than originally feared, the BBC has learned.
It is understood that the Rothko mural, Black on Maroon, could take up to 18 months to repair.
A Polish man, Wlodzimierz Umaniec, pleaded guilty to causing criminal damage last month.
Speaking on Radio 4's Today, BBC arts editor Will Gompertz said the scribbled graffiti had caused significant damage.
"There was a hope among some people that the graffiti [Umaniec] had scrawled on the corner of this painting would wipe away like a stain on a work suface," he said.
"But the ink from the pen has bled all the way through the canvas causing a deep wound, not a superficial graze.
"Because of the way in which Rothko worked, which was building his paintings layer after layer after layer in a meticulous fashion, the conservators are going to have to remove the paint layer after layer and then rebuild it.
"What looked like a funny or amusing jape to some when this was done is actually significant damage to a really major work of art, which is going to take a national institution a great deal of time and money to put right."
Restorers will have to source and apply the same range of materials employed by the artist, which could include glue, synthetic resin and egg, in an attempt to match their work with the original.
They will also have to deal with the problem that Rothko's paint is more than 50 years old and has naturally aged and changed.
A Tate spokesperson said the London gallery had always said the painting would need an extended period of conservation.
Julia Nagle, a London-based independent paintings conservator, explained that repairing a painting such as Rothko's would be a lengthy process.
"Rothko is difficult in the sense that he painted using lots of different media, which narrows down the choice of solvents that can be safely used on his paintings," she told the BBC.
"Graffiti pens are made to work outdoors and survive rain and all kinds of things, so it's something that's quite noxious that has then gone right through the paint into the canvas below."
Nagle said conservators would need to experiment on reconstructions of the original painting's damaged area.
"I've worked on an old painting from 1540 that took about six years to clean," she added. "I've also taken a year in my studio to do certain things because an awful lot of it is testing time.
"If you don't have that, you could damage something."
Umaniec, who is also known as Vladimir Umanets, pleaded guilty to criminal damage to property valued at over £5,000 at Camberwell Green Magistrates' Court on 16 October.
The Polish national, from Worthing, West Sussex, was released on conditional bail and has yet to be sentenced.