Lorri L. Jean (L) and Gina Calvelli (R) share a piece of wedding cake at a demonstration celebrating the California Supreme Court's decision overturning a ban on same-sex marriages held in West Hollywood, California on May 15, 2008. REUTERS/Phil McCarten

SAN FRANCISCO (Reuters) - A majority of registered Californian voters oppose changing the constitution of the most populous U.S. state to bar gays from marrying, according to poll released on Wednesday.

The Field Poll survey found 51 percent against approving a possible November ballot measure to prohibit gay marriage, with 43 percent in favor. A slightly differently worded question on the same issue found 54 percent opposed and 40 percent in favor.

The poll follows a state Supreme Court decision this month that barring homosexuals from marrying violated the California Constitution. Opponents of same-sex marriage have intensified efforts to put a state constitutional amendment on the ballot in November.

The poll found a strong generational gap on the issue, with those aged 18-29 approving of gay marriage by 68 percent and those 65 or older disapproving by 55 percent.

The poll found that in recent decades a growing number of Californians have approved allowing same-sex couples to marry, with 51 percent of those polled now approving, up from 44 percent in 2006 and 30 percent in 1985.

The poll of 1,052 registered Californian voters was taken from May 17-26, in the days after California Supreme Court decision, and had a sampling error of plus or minus 3.2 percentage points to 5 percentage points, depending on the question.