Leading British auction house Sotheby's has listed the 23-year-old advert as part of their Original Film Posters Online sale.The work of photographer Lorenzo Agius, the rare poster features the now iconic 'identity parade' of the film's five main protagonists.Described as near mint condition, with the colours remaining "very bright", the unframed work for the 1996 British classic measures 30 x 40 inches.Online bidding closes on 5 September with the poster initially expected to sell for between £200-£300.READ MORE: Who can remember the Trainspotting ‘It’s sh*te being Scottish’ speech?However, the current highest bidder has smashed that early estimate with a bid of £800, meaning the poster could go for a four-figure sum.Earlier this year, a previous poster signed by producer Andrew Macdonald, director Danny Boyle and screenwriter John Hodge, went for £2,500.The legendary black comedy, based upon Irvine Welsh's 1993 novel of the same name, follows the trials and tribulations of a group of Edinburgh ne'er do wells, several of whom are in the throes of heroin addiction.READ MORE: Ross Stebbing: How Trainspotting’s tragedy became a marketing ployConsidered one of the most important British films of its generation, the movie was a box office smash generating more than £48 million worldwide.A sequel, T2: Trainspotting was released in 2016 and starred all of the original main characters.Actors Robert Carlyle, Kelly Macdonald, Jonny Lee Miller, Ewen Bremner and Ewan McGregor have all gone on to enjoy further screen success.Speaking to Sotheby's in an interview ahead of the latest poster sale, Trainspotting's author Irvine Welsh explained how the book came about.He said: "I started writing because I wasn't very good at doing music. I wrote ballads but couldn't play an instrument properly. I was fed up of getting kicked out of fledgling bands, so I decided to dispense with the music and carry on with the storytelling."He added: "I only write for myself. I have no concept of an audience. It's a selfish act – I write the books I want to read."Other movie posters up for auction at Sotheby's include a 1969 advert for Easy Rider, expected to fetch upwards of £800 in the wake of the recent death of star Peter Fonda.And an original print for war flick The Dam Busters could go under the hammer for more than £12,000.But the granddaddy of the Original Film Posters sale is undoubtedly a 1933 original for King Kong. Bids start at £24,000 with the high estimate for the poster set at £50,000.