Pupils know less now about voting and politics than they did three years ago when citizenship lessons became compulsory.
The largest government study on citizenship in schools also reveals high levels of distrust of politicians among the 6,400 secondary students surveyed.
Just 18 per cent of pupils said they trusted politicians - fewer than trusted the European Union (24 per cent), teachers (52 per cent) and the police (61 per cent).
But 28 per cent said they supported a specific political party and a relatively high proportion of GCSE pupils - about three-quarters - intended to vote in elections.
Ministers introduced compulsory citizenship in secondary schools in 2002 with the aim of inspiring a greater interest in politics among young people.
Academics at the National Foundation for Educational Research tested pupils' knowledge of citizenship at more than 230 schools.
Pupils had to decide whether a series of statements such as "the minimum legal voting age is 18" and "local governments are responsible for writing local newspapers" were true or false.
The average proportion of correct answers was 60 per cent in 2002, when citizenship was introduced, but had fallen to 52 per cent last year.
The Nfer report suggested the lack of knowledge was connected to the fact that fewer pupils remembered learning about political matters than other citizenship topics such as the environment.
Only a third recalled being taught about Parliament and the Government while twice as many remembered their lessons on rights and responsibilities.
David Kerr, director of the citizenship study, said: "There is a danger that topics like voting, elections and the political process can get missed out, particularly where citizenship is taught as part of other subjects.
"It is a new area for many teachers, so they may not feel confident."
Teachers were generally positive about citizenship, and a clear majority believed that it had good effects on students' participation, skills and tolerance.
The Citizenship Foundation said the subject had always focused on providing pupils with skills rather than knowledge, but acknowledged that more factual teaching may be needed.
Listening to Young People: Citizenship Education in England is available at www.dfes.gov.uk