Belgium, France, the United Kingdom, Ireland and Netherlands cited legal costs and concerns criminal procedures could be compromised if suspects were granted lawyers for the entire length of their detainment, EUobserver reported.
The five countries filed a joint letter Friday saying the proposal would create "substantial difficulties" for investigators and could slow the process if lawyers were involved.
The extra legal costs were also of concern, they said.
"It is not possible to legislate to enhance and strengthen the rights of defendants -- either at EU or domestic level -- without also factoring in the resources and functioning of the criminal justice system," the letter reads.
However, commission spokesman Matthew Newman said ensuring a lawyer to suspects is "key ... to build confidence and mutual trust."
The proposals, announced in June, are an attempt to extend defendants' rights across the European Union.
"Citizens must know that wherever they go in the EU, they can be confident that minimum procedural rights will be guaranteed," Newman told the EUobserver.