ZURICH (Reuters) - Switzerland voted overwhelmingly to keep military conscription on Sunday, the third time citizens of the Alpine nation had rejected an attempt to reconstitute an institution regarded by many Swiss as a national 'glue'.

The neutral country, which has not fought an international war in 200 years, voted 73 percent in favor of maintaining the draft, seen as helping to bind together generations of Swiss who do not share a single language or culture.

All 23 Swiss cantons voted to keep conscription.

The result is a blow for the pacifist Group for Switzerland without an Army (GSOA), which wanted to replace compulsory military service with an all-volunteer force.

Voters rejected GSOA's attempt to scrap the army altogether in 1989, as well as 2001 referendum on replacing the military with a peace corps.

Under Swiss law, all able-bodied men are required to take part in compulsory military service between the ages of 18 and 34. Recruits complete 18-21 weeks of basic training followed by yearly refresher courses of around 19 days.

Critics say the concept is antiquated, and question the need for an army, which at roughly 150,000 troops is the same size as the Austrian, Belgian, Swedish, Norwegian and Finnish armed forces combined.

Supporters argue that relying on a solely volunteer army could compromise the nation's security. Being able to recruit the country's best IT specialists, engineers and doctors to serve is essential as the nature of warfare changes, they say.

Switzerland's neighbor Austria also backed military conscription in a referendum in January, defying a European trend to replace conscripts with exclusively professional soldiers.

(Reporting by Silke Koltrowitz and Caroline Copley; Editing by Kevin Liffey)