  This paper presents the first results of a far-infrared search for
protocluster-associated galaxy overdensities using the SPIRE instrument
on-board the {\it Herschel} Space Observatory. Large ($\sim$400 arcmin$^{2}$)
fields surrounding 26 powerful high-redshift radio galaxies ($2.0 < z < 4.1$;
$L_{\rm 500 MHz} > 10^{28.5}$ WHz$^{-1}$) are mapped at 250, 350 and 500\mic to
give a unique wide-field sample.
  On average the fields have a higher than expected, compared to blank fields,
surface density of 500\mic sources within 6 comoving Mpc of the radio galaxy.
The analysis is then restricted to potential protocluster members only, which
are identified using a far-infrared colour selection; this reveals significant
overdensities of galaxies in 2 fields, neither of which are previously known
protoclusters. The probability of finding 2 overdensities of this size by
chance, given the number of fields observed is $5 \times 10^{-4}$.
Overdensities here exist around radio galaxies with $L_{\rm 500 MHz} \gtrsim
10^{29}$ WHz$^{-1}$ and $z < 3$. The radial extent of the average far-infrared
overdensity is found to be $\sim$6 comoving Mpc.
  Comparison with predictions from numerical simulations shows that the
overdensities are consistent with having masses $> 10^{14}$Msolar. However, the
large uncertainty in the redshift estimation means that it is possible that
these far-infrared overdensities consist of several structures across the
redshift range searched.
