FILE PHOTO: A bourse trader uses a cell phone during a trading session on the trading floor at Frankfurt's stock exchange August 2, 2011. REUTERS/Ralph Orlowski

FRANKFURT (Reuters) - Older, better-educated workers accounted for the vast majority of the euro zone’s employment growth in recent years, suggesting that pension reform is forcing workers to stay in the labor market longer, the European Central Bank said on Thursday.

With Europe’s population aging rapidly, the labor force could shrink in the coming decades, placing a growing burden on government finances as the ratio of pensioners to active workers is already at a record high.

The euro zone has added about 9.6 million jobs since its 2013 employment trough. Close to three-quarters of these have been filled by workers between 55 and 74, up from less than a third in the decade before the bloc’s crisis, the ECB said in an Economic Bulletin article.

“Demographics and pension reforms are likely the main explanatory factors underlying changes in the age breakdown of employment growth,” the ECB said.

So called ‘prime age’ workers - those between 25 and 54 - accounted for only about a fifth of the employment gain during the same period, down from about two-thirds in the years preceding the crisis.

There are now about three workers for each pensioner, already a record low, and this is expected to fall to a ratio of 2 to 1 by the middle of the century, according to European Commission projections.

The ECB added that Germany accounted for a significantly larger share of employment growth than before the crisis, while the relative share of others like Italy and Spain has dropped.

The study also noted that workers who completed post-secondary education accounted for almost 80 percent of the employment growth in the recovery, up from about 60 percent in the pre-crisis years.