Lawmakers have until Thursday to decide which aides must take ObamaCare plan

House lawmakers have until Thursday to decide whether to designate staffers as "official" or "non-official," in order to determine whether they will keep their existing government insurance or be forced into ObamaCare.

Fox News has learned that rank-and-file members were briefed Tuesday by House Administration Committee Chairwoman Candice Miller during a closed-door meeting and informed of the Thursday deadline.

So what qualifies as an "official" staffer?

According to internal rules, House employees whose salaries are paid entirely from a "Members Representational Allowance" will be determined “official office” staff and eligible only for ObamaCare insurance through the District of Columbia-run health care exchange.

Those whose salaries are not paid entirely from the representational allowance will be considered non-official and can continue to get insurance through a standard federal plan.

Miller, R-Mich., advised the lawmakers based on guidance from the Chief Administrative Office -- the non-partisan, non-legislative office that handles business and support services for House members, officers and staff. The guidance is based on Obama administration-issued regulations on “the eligibility of certain House employees” to continue to get health insurance through the Federal Employees Health Benefits Program, following ObamaCare being signed into law, according to a CAO document obtained by Fox News.

“This is a really good way to start a civil war,” a Republican lawmaker told Fox News.

The determination is based on a staffer’s pay as of Oct. 15, and the CAO will make the determination if lawmakers do not submit the paperwork by 5 p.m. Thursday.

Right now, all congressional employees are eligible for the employer-contribution for health care -- a perk Republicans tried to eliminate during the recent partial government shutdown for those who would end up on the exchanges.

Fox News' Chad Pergram contributed to this report.