Airline worker Brian Baggott could never afford an original painting or print by David Hockney.

But he did the next best thing and collected posters advertising the Bradford-born artist's exhibitions instead.

Now part of his unrivalled hoard, chronicling the career of the world-famous artist, is set to fetch a small fortune when it goes under the hammer at Christie's in London.

The auction of more than 100 images, painstakingly assembled over a 25-year period, is expected to sell for around £75,000.

Most of the billboard posters were designed by Hockney to advertise exhibitions of his original works.

Mr Baggott, a long-time friend of the artist, waited until they had served their promotional purpose in shop windows and picked them up for next to nothing.

Highlights include silkscreen images produced for Igor Stravinsky at the Metropolitan Opera, New York, in 1981 which are expected to fetch up to £1,500 .

A 1992 Chelsea Arts Ball poster and one in 1985 for the magazine Vogue Paris should realise £600 each. Others, with estimates from £200 upwards, were produced to promote exhibitions in New York, Los Angeles, Australia and a display of Hockney's work at Salts Mill in Saltaire.

Mr Baggott, 53, from Fulham, said: "I had known David since the late 1960s but could never afford anything original.

"Someone bought me a poster for a Christmas present and it set me on the path to collecting Hockney posters and catalogues."

As his collection grew, the airline worker found he had some posters that the artist was not even aware had been produced.

He said: "Copyright laws in some places are crazy. I used to pop into David's offices in London and tell them about posters I'd acquired and in return they would give me some others."

Christie's specialist Nicolette White said: "It is possible to visually trace Hockney's prolific career through his posters. One reason for their popularity is that they fit perfectly into the modern interior." The posters, many signed by the 61-year-old artist, will go under the hammer on March 25.

Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.