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We report the discovery of a peculiar L dwarf from the UKIDSS LAS, ULASJ222711-004547. The very red infrared photometry (MKO J-K = 2.79$\pm$0.06, WISEW1-W2 = 0.65$\pm$0.05) of ULAS J222711-004547 makes it one of the reddest browndwarfs discovered so far. We obtained a moderate resolution spectrum of thistarget, and classify it as L7pec, confirming its very red nature. Comparison totheoretical models suggests that the object could be a low-gravity L dwarf witha solar or higher than solar metallicity. Nonetheless, the match of such fitsto the spectrum is rather poor and this and other peculiar red L dwarfs posenew challenges for the modeling of ultracool atmospheres. We determined theproper motion of ULAS J222711-004547 using the data available in theliterature, and we find that its kinematics do not suggest membership of any ofthe known young associations. We show that applying a simple de-reddening curveto its spectrum allows it to resemble the spectra of the L7 spectroscopicstandards. Given the negligible interstellar reddening of the field containingour target, we conclude that the reddening of the spectrum is mostly due to anexcess of dust in the photosphere of the target. De-reddening the spectrumusing extinction curves for different dust species gives surprisingly goodresults and suggests a characteristic grain size of $\sim$0.5 $\mu$m. We showthat by increasing the optical depth, the same extinction curves allow thespectrum of ULAS J222711-004547 to resemble the spectra of unusually blue Ldwarfs and slightly metal-poor L dwarfs. Grains of similar size yield very goodfits when de-reddening other unusually red L dwarfs in the L5 to L7.5 range.These results suggest that the diversity in near infrared colours and spectraseen in late-L dwarfs could be due to differences in the optical thickness ofthe dust cloud deck.

The extremely red L dwarf ULAS J222711$-$004547 - dominated by dust