Image copyright Getty Images Image caption US President Donald Trump pays close attention to who's on the cover of Time magazine.

US magazine publisher Time Inc will be bought by rival media company Meredith Corporation for $2.8bn (£2.1bn).

The deal is backed by the billionaire brothers Charles and David Koch, who are known for supporting economically conservative causes.

Meredith is an Iowa-based publisher and broadcaster which has made two previous unsuccessful bids for Time.

Time has struggled with declining advertising revenues since it was spun off from Time Warner in 2014.

In addition to Time, the company also publishes People, Sports Illustrated, Entertainment Weekly and Fortune magazines.

In the UK, Time Inc's titles include Wallpaper, the NME, Horse & Hound, World Soccer, Uncut, Women's Own, Ideal Home, and Anglers' Mail, among others.

In November, Time's third quarter revenue slipped 9.5% to $679m, marking the sixth straight quarter the publisher fell short of analysts expectations.

"This all-cash transaction, and the immediate, certain value it provides, is in the best interests of the company and its shareholders," Time's Chairman John Fahey said in a statement.

Meredith owns print publications Family Circle and Better Homes and Gardens as well as local television stations across the US.

The company said the deal would give it nearly $700m in digital advertising revenues and an "unmatched reach" to the valuable millennial customers, in their twenties and thirties.

'Political activism'

As part of the deal, Meredith has secured $650m from the Koch brothers' private equity arm, Koch Equity Development.

The two brothers run Koch Industries, which is one of the world's largest privately-owned companies, with interests ranging from pipelines to paper towels.

According to the Koch Industries website, "to millions of Americans, the words 'Koch brothers' and 'political activism' go hand-in-hand".

They've long championed economically conservative candidates and causes, but they've also pushed criminal justice reform and made large donations to the American Civil Liberties Union.

Image copyright Getty Images Image caption Previous interest from the Koch brothers in the LA Times prompted protests from the public who feared editorial interference

Their involvement in a media deal is likely to raise concerns that they'll use their investment to wield influence.

A suggestion they might attempt to acquire the Los Angeles Times and Chicago Tribune in 2013 sparked petitions and protests from readers concerned about their intentions.

However, Meredith said the Koch brothers won't be given a seat on the board of Time Inc, and won't have any managerial or editorial input.

Person of the year

Meredith says the acquisition of Time Inc will give its brands a readership of 135 million and paid circulation of nearly 60 million.

"We believe this acquisition represents a transformative and financially compelling growth opportunity for Meredith Corporation and will increase shareholder value over time," Meredith's chief executive Stephen Lacy said in a statement.

Even if Time Inc has struggled in recent years, its eponymous magazine still holds significant cultural influence.

An example is its "person of the year" issue, which is now in its 90th year.

The issue recognises the person who "for better or for worse... has done the most to influence the events of the year".

Past recipients have included almost every sitting US president, Queen Elizabeth, Pope John Paul II and Mahatma Gandhi.

More controversially, Joseph Stalin and Adolf Hitler also both received the recognition.

This year's choice made headlines last week, after US President Donald Trump claimed he was expected to be given the title for a second consecutive year.

Time magazine disputed the president's claim, and said it did not comment on the cover story before publication.