A letter to Finance Secretary Mark Drakeford warns they may be forced to cut frontline police budgets or lower training standards below England's.
They are particularly concerned that the £2m they pay in the apprenticeship levy is not spent on police training.
The Welsh Government said the levy does not cover Treasury funding cuts.
North Wales commissioner Arfon Jones, who chairs the All-Wales Policing Group, said in the letter that Welsh forces were "now at a serious financial disadvantage and in future will be disadvantaged in terms of training compared to our English counterparts".
He warned that Wales faced a bill of £8.9m for the Police Education Qualifications Framework (PEQF) by 2020-21.
While English forces only had to meet 8% of training costs with the rest covered by the apprenticeship levy, Mr Jones said forces in Wales faced paying 95% to 100% from their own budgets.
A shortfall of £10m would be equivalent to 200 fewer officers in Wales, he added.
Welsh forces had already lost 2,500 police officers and staff since 2010, Mr Jones said, "equivalent to the entire resources of North Wales Police".
"Under such pressures, Welsh forces might have to lower the standard of training and it would be regrettable to break away from the uniform standards that are currently in place," Mr Jones said.
"Being forced to fund the training from existing resources would inevitably have an impact on frontline policing."
A Welsh Government spokeswoman said ministers had written to Home Secretary Amber Rudd about the issue of police training.
"While we are supportive of the PEGF in principle, we have been clear that responsibility for operational policing matters, including training, rests solely with the Home Office and it should fund training until policing is devolved," she said.
As for the apprenticeship levy - 0.5% payable on annual wage bills of more than £3m - the spokeswoman said it was a UK government employment tax "which directly conflicts with areas of devolved responsibility".
As skills policy is devolved, the money is not necessarily spent on apprenticeships in Wales.
Instead, the Welsh Government is entitled to a population-based share of the proceeds under the UK Treasury's Barnett formula, to be spent as ministers in Cardiff see fit.
The spokeswoman said such money had been "largely offset by cuts made elsewhere, meaning there is no significant new money to invest in services as a result of the apprenticeship levy".
A Home Office spokesman said: "All police forces pay the apprenticeship levy, and in Wales those funds are passed back to the Welsh Government through an arrangement agreed with HM Treasury."
Wales could have 200 fewer police officers than it should by 2021 due to a shortfall of up to £10m in training cash, police chiefs have claimed.