The invention relates to a prepolymer process for preparing a high resilience flexible polyurethane foam by reacting a polyisocyanate composition comprising at least 80% by weight of 4,4′-MDI and having an NCO value below 20% with a polyol composition comprising b1) an EO-rich polyol; b2) a polyol with an EO content between 20-50% and a primary hydroxy content of at least 50%; and b3) a polyol with an EO content between 10-20% and a primary hydroxy content of at least 50%; according to the following proportions: b1:40-90 wt %, b2:5-60 wt %, b3:0-20 wt %. The invention also relates to reaction systems comprising the above components.