  Aims. We present a method, named photo-type, to identify and accurately
classify L and T dwarfs onto the standard spectral classification system using
photometry alone. This enables the creation of large and deep homogeneous
samples of these objects efficiently, without the need for spectroscopy.
Methods. We created a catalogue of point sources with photometry in 8 bands,
ranging from 0.75 to 4.6 microns, selected from an area of 3344 deg^2, by
combining SDSS, UKIDSS LAS, and WISE data. Sources with 13.0 < J < 17.5, and Y
- J > 0.8, were then classified by comparison against template colours of
quasars, stars, and brown dwarfs. The L and T templates, spectral types L0 to
T8, were created by identifying previously known sources with spectroscopic
classifications, and fitting polynomial relations between colour and spectral
type. Results. Of the 192 known L and T dwarfs with reliable photometry in the
surveyed area and magnitude range, 189 are recovered by our selection and
classification method. We have quantified the accuracy of the classification
method both externally, with spectroscopy, and internally, by creating
synthetic catalogues and accounting for the uncertainties. We find that,
brighter than J = 17.5, photo-type classifications are accurate to one spectral
sub-type, and are therefore competitive with spectroscopic classifications. The
resultant catalogue of 1157 L and T dwarfs will be presented in a companion
paper.
