El Greco's The Baptism of Christ dates from 1567-70 (Agencies)

A 16th Century painting by artist El Greco, valued at ￡500,000, has been discovered in an envelope in Spain.

The Spanish family that has owned the oil-on-wood portable altarpiece , The Baptism of Christ, since the mid-19th Century, did not know what they had.

It was only discovered when they took it to a routine Christie's valuation.

The auctioneers said it was a "wonderful, vibrant work" and came as a "complete surprise". It will be sold by Christie's in London on 8 December.

El Greco, born in Crete, had a unique style, characterised by elongated forms and bright colours, that inspired 20th Century painters such as Cezanne, Picasso and Jackson Pollock.

The Baptism of Christ measures 23.7cm by 18cm and was found when a man living in western Spain responded to an advertisement offering art valuations, Christie's said.

"He obviously didn't realise what he had," a spokesman said. "It had been in his family since the mid-19th Century and simply been ignored."

The auction house says it dates from El Greco's time in Venice, from 1567-70, and is an extremely rare example of his work from that period.

"To discover a painting of such beauty and importance came as a complete surprise," according to the auctioneers.

"This painting originates from a particularly exciting moment in El Greco's career, when the artist was still developing what became his distinctive style."

The news of its discovery comes just a week after another "lost" El Greco work was put on display in Poland.

Ecstasy of St Francis was discovered in a small church in east Poland during a routine check of historical artefacts in 1964. 

(Agencies)