Twenty Massachusetts cities including Boston and Cambridge filed suit together yesterday to force a recount of the 1980 census, because officials said they fear the population decreases reported threaten federal funding and state representation.

Cambridge may also file its own suit, John Hixson, technical specialist of the Community Development Department, said yesterday, explaining the city "might have areas of difficulty which would not be taken care of in the state suit."

City officials agreed yesterday that Cambridge could lose "substantial" federal funding in the forms of CETA and other employment aid grants, although they would not provide exact figures. State losses of Congressional seats would also hurt the city indirectly.

Cambridge has lost 5010 persons since 1970, a 5 percent drop, preliminary Census Bureau figures show. Boston, Watertown, and Somerville report losses of more than 12 per cent.

But state and city officials, including Boston mayor Kevin H. White and Attorney General Francis X. Bellotti, have questioned the figures, saying the census undercounted non-English-speaking minorities and residents of poor neighborhoods.

Students Undercounted

Census officials also undercounted many Cambridge-area students who didn't know they were officially city residents, Tim Reidy, acting director of Cambridge's CETA program, said this week.

A federal judge in Detroit last week invalidated census results on similiar grounds, and Cambridge officials said they are optimistic that they will win. "We have as good a chance as anyone of winning," Hizson said.

But Boston City "Councilman Albert O'Neal predicted yesterday the suit wil fail, saying the census is accurate because "people are moving out of the cities all the time. They are being forced out by crime, forced busing, and constant tax increase.