  The observed number counts of high-redshift galaxy candidates have been used
to build up a statistical description of star-forming activity at redshift z >~
7, when galaxies reionized the Universe. Standard models predict that a high
incidence of gravitational lensing will probably distort measurements of flux
and number of these earliest galaxies. The raw probability of this happening
has been estimated to be ~ 0.5 percent, but can be larger owing to
observational biases. Here we report that gravitational lensing is likely to
dominate the observed properties of galaxies with redshifts of z >~ 12, when
the instrumental limiting magnitude is expected to be brighter than the
characteristic magnitude of the galaxy sample. The number counts could be
modified by an order of magnitude, with most galaxies being part of multiply
imaged systems, located less than 1 arcsec from brighter foreground galaxies at
z ~ 2. This lens-induced association of high-redshift and foreground galaxies
has perhaps already been observed among a sample of galaxy candidates
identified at z ~ 10.6. Future surveys will need to be designed to account for
a significant gravitational lensing bias in high-redshift galaxy samples.
