The Scottish party is embroiled in an internal battle over proposals to devolve the tax to Scotland.
Scottish Labour leader Johann Lamont is expected to unveil the policy, expected to form part of a devolution offer ahead of September's independence referendum, before the party's conference in Perth next month. But a number of her own MPs are threatening to boycott the event in protest, warning it would lead to cuts in the amount of money Scotland receives from the UK and in the number of MPs it sends to Westminster.
Their position appeared to have the backing of senior Welsh Labour politicians.
But yesterday, Welsh Labour assembly members voted against restrictions they believe would limit Welsh ministers' ability to vary income tax rates. The issue centres on the so-called 'lockstep', which means that any change to one tax band would apply to them all.
Labour First Minister of Wales Carwyn Jones said: "The lockstep is useless, it makes no sense at all."
A senior Scottish Labour source said: "We work closely with Carwyn and are anxious that the new (devolution) settlement is as even as possible across the UK.
"We welcome this development."