Russian art restorers on Friday unveiled a recently discovered portrait of the last Tsar Nicholas II, almost a century after it was hidden behind a giant painting of his Bolshevik foe Lenin.

A team of art restorers led by Tatiana Potseluyeva painstakingly uncovered the tsar's image over the last three years.

The portrait of Nicholas -- shot by the Bolsheviks in 1918 -- was hidden under a layer of water-soluble paint, suggesting it was meant to be preserved and eventually found.

"The ceremonial portrait painted by Ilya Galkin in 1896 was hidden for almost 90 years on the back of another portrait -- depicting Lenin," Potseluyeva told AFP.

Galkin, a little-known artist who painted several portraits of tsars, died in 1915 before the October Revolution of 1917.

The portrait of Lenin wearing a peaked cap and standing in front of Saint Petersburg's Peter and Paul fortress was painted by another artist, Vladislav Izmailovich, in 1924, Potseluyeva said.

Since then, the giant portrait measuring four by three metres (13 by 10 feet) has hung in the assembly hall of a school in the historic centre of Saint Petersburg.

The painting was damaged in the 1970s but restoration experts from the city's Shtiglits Arts and Craft Academy only began restoring it in 2013, leading to the discovery of the hidden portrait.

"We were really surprised!" recalled Potseluyeva, adding that the first detail they discovered was the ornate carpet on which the tsar was standing.

- Taking a risk -

The Lenin portrait painter Izmailovich, who died in 1959, apparently took pains to preserve the work of his predecessor and camouflage it, working in the year when the long-ailing Bolshevik leader died and Stalin cemented his control of the regime.

"Normally you paint over the old canvas, destroying the previous image," Potseluyeva said.

"Here the painter kept it and made it disappear under a coat of water-soluble paint, before painting on the reverse" of the canvas, she said.

"It seems that he hoped that one day the portrait of Nicholas II would be discovered," she said.

Izmailovich studied in Paris, Rome and Berlin before the Revolution and became known for frescoes and portraits.

After 1917, he was one of the first to paint Lenin from life. He also painted scenes of revolutionary history and taught art.

While his motive for hiding the tsar's portrait may never be known, experts said that he ran a risk by doing so. 

"By keeping the tsar's portrait, Vladislav Izmailovich risked a lot at that time," said the acting head of the Shtiglits Academy, Vasily Kichedzhi.

The Bolsheviks pulled down statues and removed the tsars' emblem from buildings after taking power.

The double-sided canvas with both paintings will be put on show to the public at the end of the month at the Academy.

In neighbouring Ukraine, the pro-Western authorities have launched a campaign to topple all the Lenin statues, prompting fury in Moscow, where Lenin's embalmed body is still on display in the Mausoleum on Red Square.