  We present the results of an extensive search for periodic and irregular
variable pre-main sequence (PMS) stars in the young (2-4 Myr) open cluster NGC
2664, based on photometric monitoring using the Wide Field Imager (WFI) on the
2.2m telescope on La Silla (Chile). In total, about 10600 stars with I_c
magnitudes between 9.8mag and 21mag have been monitored in our 34x33 arcmin
field. Time series data were obtained in the I_c band in 44 nights between Dec.
2000 and March 2001; altogether we obtained 88 data points per star. Using two
different time series analysis techniques (Scargle and CLEAN) we found 543
periodically variable stars with periods between 0.2 days and 15 days. Also,
484 irregular variable stars were identified using a chi-squared-test. In
addition we have carried out nearly simultaneous observations in V, R_c and a
narrow-band Halpha filter. The photometric data enable us to reject background
and foreground stars from our sample of variable stars according to their
location in the I_c vs. (R_c-I_c) colour-magnitude and (R_c-Halpha) vs.
(R_c-I_c) colour-colour diagram. We could in the end identify 405 periodically
variable and 184 irregular variable PMS stars as cluster members using these
two different tests. In addition 35 PMS stars for which no significant
variablilty were detected could be identified as members using an Halpha
emission index criterion. This yields to a total of 624 PMS stars in NGC 2264,
of which only 182 were known before. Most of the newly found PMS stars are
fainter than I_c = 16 and of late spectral type (>M2). (abridged)
