More than 1,400 researchers have signed an online letter backing the principles of Plan S, the bold open-access initiative led by research agencies who say that, by 2020, papers resulting from their funding should be immediately free to read on publication.

The petition, launched on 28 November, comes as scientists continue to debate the pros and cons of the European-led plan, which was announced in September and is now supported by 16 national science funders and charities.

“The only way to achieve universal open access to the scientific literature is for research funders to require it of their grantees. They're finally doing it, but are taking a LOT of flak,” tweeted the letter’s organizer, Michael Eisen, a geneticist at the University of California, Berkeley, and a long-time proponent of open-access (OA) publishing. (The letter does not specifically name Plan S, but quotes its cornerstone policy.)

Some of that flak came in a letter published three weeks earlier , co-ordinated by Lynn Kamerlin, a biochemist at Uppsala University in Sweden. Kamerlin’s letter, which itself has gathered more than 1,400 signatures, called Plan S “a serious violation of academic freedom”, because its terms would prevent researchers from publishing where they wanted.

Eisen’s letter directly rebuts the academic-freedom argument. While funder mandates may “superficially limit publishing options in the short term”, the letter says, they will “ultimately lead to a system that maximizes the reach of scholarship and its value to the research community and public.”

Eisen says he thinks it is “laughable and dangerous” to suggest that funder mandates impinge on academic freedom when at the same time, he argues, the current publication system effectively “forces researchers to publish in high impact-factor journals and the community to spend billions of dollars on subscriptions”.

In response, Kamerlin says that she also supports open access, but prefers efforts such as those of Denmark’s science ministry, which has said that researchers can choose to publish behind paywalls as long as they archive their papers online within 12 months.

Since Kamerlin’s letter appeared, Plan S funders have clarified that their initiative will not be as restrictive as some had worried it might be.

As originally written, it appeared that the plan might bar publishing in ‘hybrid’ journals — which keep work behind paywalls but offer an open-access option for a fee.

But on 27 November, funders said that as long as researchers archive their papers immediately and openly online, they can publish in hybrid journals — although Plan S agencies will not pay publishing fees to do so.

The plan is now out for public consultation until 1 February. One early analysis, by Jeroen Bosman and Bianca Kramer, two librarians and scholarly communication researchers at Utrecht University in the Netherlands, counts at least nine routes by which researchers can comply with the initiative.