Stable modified polymer polyol dispersion of polymeric particles in a liquid polyol mixture, the stable modified polymer polyol dispersion is obtainable by a process comprising the preparation of polymeric particles, in the presence of the liquid polyol mixture, by reacting in the liquid polyol mixture and under polymerisation conditions ▪ at least one compound bearing at least one basic-nitrogen atom a.1) characterized by the presence in the molecule of at least one hydrogen atom attached to a nitrogen atom; with ▪ a mixture of compounds bearing a phosphorus atom a.2) comprising [bis(hydroxymethyl)phosphanyl]methanol, a tetrakis(hydroxymethyl) phosphonium salt (THPX), [bis(hydroxymethyl)phosphoryl]methanol (THPO), ({[bis(hydroxymethyl)phosphanyl]methoxy}methanol (mTHP), {bis[(hydroxymethoxy)methyl]phosphanyl}methanol (dTHP), ({bis[(hydroxymethoxy)methyl]phosphanyl}methoxy)methanol (tTHP)), bis(hydroxymethyl)phosphorylmethoxymethanol (mTHPO), {bis[(hydroxymethoxy)methyl]phosphoryl}methanol (dTHPO) and ({bis[(hydroxymethoxy)methyl]phosphoryl}methoxy)methanol (tTHPO); or ▪ the mixture of condensation products a.3) resulting from the reaction of the mixture of compounds bearing a phosphorus atom a.2) as described above and at least one compound bearing at least one basic nitrogen atom a.1); wherein the compound bearing at least one basic-nitrogen atom a.1) is selected from - melamine, trimethylol melamine; - the compounds of formula or formula; and - mixtures thereof; wherein the compounds of formula are as follows R 1 -NH 2 wherein R 1 is selected from -H, -(C 1 -C 3)Alkyl, (C 1 -C 3)Alkyl-CO-, NH 2 -CO-, -(CH 2) n -NH 2, -CN, -NH 2, -SO 2 NH 2, cyclohexyl, phenyl, -SO 2 OH and -NHCONHNH 2; wherein n is an integer from 1-18; and the compounds of formula are as follows: wherein R 2 is selected from NH=C<, O=C<, and S=C<; and R 3 is selected from H, -CN, -NH 2, -CONH 2, -CONHCONH 2, and -CONHCONHCONH 2; wherein the liquid polyol mixture comprises at least one base polyol and at least one monofunctional polyetheramine, the monofunctional polyetheramine having a propylene oxide content from 30-99 wt%. ▪ at least one compound bearing at least one basic-nitrogen atom a.1) characterized by the presence in the molecule of at least one hydrogen atom attached to a nitrogen atom; with ▪ a mixture of compounds bearing a phosphorus atom a.2) comprising [bis(hydroxymethyl)phosphanyl]methanol, a tetrakis(hydroxymethyl) phosphonium salt (THPX), [bis(hydroxymethyl)phosphoryl]methanol (THPO), ({[bis(hydroxymethyl)phosphanyl]methoxy}methanol (mTHP), {bis[(hydroxymethoxy)methyl]phosphanyl}methanol (dTHP), ({bis[(hydroxymethoxy)methyl]phosphanyl}methoxy)methanol (tTHP)), bis(hydroxymethyl)phosphorylmethoxymethanol (mTHPO), {bis[(hydroxymethoxy)methyl]phosphoryl}methanol (dTHPO) and ({bis[(hydroxymethoxy)methyl]phosphoryl}methoxy)methanol (tTHPO); or ▪ the mixture of condensation products a.3) resulting from the reaction of the mixture of compounds bearing a phosphorus atom a.2) as described above and at least one compound bearing at least one basic nitrogen atom a.1); wherein the compound bearing at least one basic-nitrogen atom a.1) is selected from - melamine, trimethylol melamine; - the compounds of formula or formula; and - mixtures thereof; - melamine, trimethylol melamine; - the compounds of formula or formula; and - mixtures thereof; wherein the compounds of formula are as follows R 1 -NH 2 wherein R 1 is selected from -H, -(C 1 -C 3)Alkyl, (C 1 -C 3)Alkyl-CO-, NH 2 -CO-, -(CH 2) n -NH 2, -CN, -NH 2, -SO 2 NH 2, cyclohexyl, phenyl, -SO 2 OH and -NHCONHNH 2; wherein n is an integer from 1-18; and the compounds of formula are as follows: wherein R 2 is selected from NH=C<, O=C<, and S=C<; and R 3 is selected from H, -CN, -NH 2, -CONH 2, -CONHCONH 2, and -CONHCONHCONH 2; wherein the liquid polyol mixture comprises at least one base polyol and at least one monofunctional polyetheramine, the monofunctional polyetheramine having a propylene oxide content from 30-99 wt%.