Image copyright Getty Images Image caption The law known as Obamacare is a key part of the president's legacy

The cost of healthcare insurance in the US under the Affordable Care Act is expected to rise by an average of 25% in 2017, according to the government.

About one in five consumers will also only be able to pick plans from a single insurer, it said.

But it said federal subsidies will also rise, and about 70% of people will find plans for less than $75 (£61) a month.

Republican nominee Donald Trump vowed on Tuesday to repeal the law, which is known as Obamacare.

It is a major part of President Barack Obama's legacy, and his signature piece of legislation.

"Obamacare is just blowing up," said Mr Trump, who has promised his own plan would deliver "great healthcare at a fraction of the cost".

The enactment of the Affordable Care Act (ACA) in 2010 mandated that every American had to purchase private insurance, and prohibited insurers from turning away the sick. It also provided subsidies.

But Republicans want to repeal it.

According to the report from the Department of Health and Human Services, for a 27-year-old consumer, in the prime age group sought by insurers, the average monthly premium for a benchmark plan would be $302 next year, up from $242 this year.

The average increase of 25% in benchmark premiums on the federal exchange compares with increases of 2% in 2015 and 7% this year.

Democratic presidential nominee Hillary Clinton has said she supports the Affordable Care Act, but has denounced "skyrocketing out-of-pocket health costs", saying the federal government should have the power to block or modify unreasonable rate increases.

Analysis: Anthony Zurcher, BBC News, Washington

It's only for some people in some places, but health insurance premiums are going up - and it's generating unpleasant headlines for Hillary Clinton and the Democrats.

Americans with government-provided insurance subsidies will be fine. Those with employer-sponsored plans are also OK. But people caught in between - small-business owners and the self-employed - will take it on the chin in states like Arizona (premiums up 116%), Oklahoma (69%), Tennessee (63%) and Minnesota (59%).

The unsubsidised portion of the government-run health insurance marketplace was what Bill Clinton notably called "the craziest thing in the world" earlier this month. Donald Trump and the Republicans tried to make hay out of his remarks at the time - and now they have solid numbers to make their case.

Mrs Clinton continues to be in the awkward position of defending Obamacare, pointing out that it has decreased the rate of US healthcare spending growth and provided increased coverage for Americans, while conceding the law needs to be improved.

If she's elected, she may try her best to push through a legislative repair job, but Republicans in Congress may be happy to watch the programme continue to struggle - and, perhaps, take her down with it.

Follow Anthony on Twitterhere.

Obamacare has already faced two major challenges in the US's highest court.

Congressional Republicans have voted more than 50 times to undo the law.

Unlike in many other western countries, the US does not have a single-payer healthcare system. Private companies, rather than the US government, provide health insurance for US citizens.

Following the enactment of the ACA in 2010, states were given the option of establishing their own healthcare exchanges - online marketplaces for citizens to buy health coverage.

Citizens in states that refused to establish exchanges could shop for coverage on a federal exchange.

However, most Americans receiving subsidies purchase healthcare through the federal exchange, after many states decided not to set up their own marketplaces.

Obamacare by the numbers

citizens in 37 states depend on federal subsidies to make healthcare affordable

depend on federal subsidies to make healthcare affordable only 13 states and Washington, DC have established their own exchanges

have established their own exchanges over 10m people have purchased coverage through one of the new exchanges - federal or state

have purchased coverage through one of the new exchanges - federal or state on average, the federal government provides a $272 (£173) monthly subsidy

Source: Reuters