An opinion poll has found that a majority of Americans oppose letting employers, based on their religious views, exclude certain contraceptives from workers' insurance coverage.

The Reuters/Ipsos opinion poll was conducted ahead of a US Supreme Court decision expected tomorrow.

The nine-member court will weigh whether for-profit corporations may raise religious objections to a mandate in President Barack Obama's signature 2010 healthcare law that their insurance cover contraceptives.

The law, known as Obamacare, was itself broadly upheld by the Supreme Court in 2012.             

The poll asked whether employers should be able to choose what forms of contraceptives their health plans provide based on their religious beliefs.

Of those responding, 53% disagreed and 35% agreed. Of those surveyed, 12% said they did not know.

The justices will sit at 3pm Irish time tomorrow for the final day of their nine-month annual term.

In the case, two family-owned companies, Hobby Lobby and Conestoga Wood Specialties, challenged the insurance requirement for certain employee birth control devices and methods as a violation of a 1993 religious-freedom law.

The Oklahoma based arts-and-crafts retailer Hobby Lobby is controlled by evangelical Christians, and the Pennsylvania-based cabinet-manufacturer Conestoga Wood Specialties is owned by Mennonites.

The healthcare law already exempts churches and religious-run entities from the contraceptive mandate.

The companies, and others involved in related lawsuits, do not oppose every type of birth control. Some object only to emergency contraceptive methods, such as the "morning-after" pill, which they view as akin to abortion.

The Obama administration contends for-profit corporations, even closely held ones, do not exercise religious rights as individuals do and are not covered by the 1993 Religious Freedom Restoration Act.

During oral arguments in March, the justices seemed split along ideological lines, with the five conservative justices suggesting they might be ready to rule that certain for-profit entities have the same religious rights to object to federal requirements as individuals do.

In a 2010 case known as Citizens United, the five-justice conservative bloc enhanced corporate free speech rights when it struck down campaign finance regulation.

The Reuters/Ipsos poll of 10,693 people was conducted between 28 April and 20 June.

It found that 40% strongly disagreed and 13% somewhat disagreed with the idea that employers should be able to choose what forms of contraceptives their health plans provide based on their religious beliefs.

It found that 20% strongly agreed and 15% somewhat agreed with the idea.

The poll has a credibility interval of plus or minus 1.1 percentage points.

The administration contends that if the court rules for the companies, the decision could lead to lawsuits by corporations against other federal requirements, including minimum-wage and Social Security tax laws.

A company might also object, some liberal justices suggested during oral arguments, to providing coverage for vaccinations.