The Prince George's County Council yesterday approved a referendum on reducing council membership from 11 to nine. If approved by county voters this Nov. 4, the measure would go into effect with the 1982 elections.

The all-Democratic council was under pressure from county Republicans and Democrats unallied with the county's Democratic organization who have mounted a petition drive to reduce the size of the council to nine and have all of its members elected from individual districts rather than at large.

The petition drive, aimed at getting its supporters' proposal on the referendum ballot in November, is an attempt to loosen the strong hold of the Democratic Party organization on the county's elective offices, in particular the council.

The present council was elected under a law requiring five of its members to live in separate election districts while all 11 were listed on the county-wide ballot.

A new law that will govern the 1982 elections -- unless supplanted as a result of a referendum -- will require election of five council members solely by the voters of the individual districts they serve, while six others would be elected at large.

If the referendum the council approved yesterday wins at the polls in November, four council members will be chosen from individual districts and five at large.

The group seeking to have all council members elected from districts can also have its referendum question listed on the fall ballot if it collects 10,000 signatures. It claims to have 7,000 already.

Election of council members by districts, petition drive supporters say, would be less expensive and therefor would make it easier for persons without the backing of party leaders or who are not included on the Democratic slate to run for office. In the 1978 county elections only one person was able to beat a candidate selected by the Democratic Party leaders for a county council seat.

Council members have charged that the petition drive is a partisan effort by the Republican Party, including County Executive Lawrence J. Hogan, and disgruntled Democrats, that is intended to make it easier for Republicans to win election in the heavily Democratic county. In addition, they say, election by district would increase parochialism on the council to the detriment of the county.