Government policy advisers, crown prosecutors and other top civil servants are among members of the First Division Association who voted Monday to join the strike over plans to increase their retirement age.
Five other unions representing government employees had already voted in favour of the 23 March strike, which is expected to involve 1.5 million workers and cripple public services.
Members of the FDA, who includes senior government managers, tax inspectors, economists, statisticians and government lawyers, have not taken industrial action since 1981.
Unions claim there is widespread disapproval among workers over government plans to reform the pension scheme for civil servants, including increasing the retirement age from 60 to 65.
Talks aimed at averting the strike action are expected to be held between union leaders, employers and Deputy Prime Minister John Prescott later this week.