By virtue of an office or position.

Roget 494: real, actual etc. (existing) 1; veritable, true; right, correct; certain etc. 474; substantially true, categorically true, definitively true &c.; true to the letter, true as gospel; unimpeachable; veracious etc. 543; unreconfuted^†, unconfuted^†; unideal^†, unimagined; realistic.    exact, accurate, definite, precise, well-defined, just, just so, so; strict, severe; close etc. (similar) 17; literal; rigid, rigorous; scrupulous etc. (conscientious) 939; religiously exact, punctual, mathematical, scientific; faithful, constant, unerring; curious, particular, nice, delicate, fine; clean-cut, clear-cut.    verified, empirically true, experimentally verified, substantiated, proven (demonstrated) 478.    rigorously true, unquestionably true.    true by definition.    genuine, authentic, legitimate; orthodox etc. 983.1; official, ex officio.    pure, natural, sound, sterling; unsophisticated, unadulterated, unvarnished, unalloyed, uncolored; in its true colors; pukka^†.    well-grounded, well founded; solid, substantial, tangible, valid; undistorted, undisguised; unaffected, unexaggerated, unromantic, unflattering.    ... show more

Roget 737a: regal, sovereign, governing; royal, royalist; monarchical, kingly; imperial, imperiatorial^†; princely; feudal; aristocratic, autocratic; oligarchic etc. n.; republican, dynastic.    ruling etc. v.; regnant, gubernatorial; imperious; authoritative, executive, administrative, clothed with authority, official, departmental, ex officio, imperative, peremptory, overruling, absolute; hegemonic, hegemonical^†; authorized etc. (due) 924.    [pertaining to property owned by government] government, public; national, federal; his majesty's [Brit.], her majesty's; state, county, city, etc. N.. ... show more

By virtue of position:
— The president sat on the committee ex officio.

synonym: by right of office.

Roget 924: duly, ex officio, de jure [Lat.]; by right, by divine right; jure divino [Lat.], Dei gratia [Lat.], in the name of.