The Wellcome Trust plans to withhold a portion of grant money from scientists who do not make the results of their work freely available to the public, in a move that will embolden supporters of the growing open access movement in science. In addition, any research papers that are not freely available will not be counted as part of a scientist's track record when Wellcome assesses any future applications for research funding.

The trust is the second largest medical research charity in the world, spending more than £600m on science every year. Its director, Sir Mark Walport, is a firm supporter of open access to scientific research as a way to ensure scientific results are available to as wide a range of people who might benefit from them as possible. He has said that publishing research papers should be considered a cost of a research project in the same way as a piece of lab equipment.

His calls have echoed growing unrest from academics around the world who want to release the results of public and charity-funded scientific research from behind the paywalls of academic publishers, which charge UK universities hundreds of millions of pounds between them to access the information.

Many supporters of open access advocate the "gold" model, where researchers pay an upfront fee to a journal for their paper to be made available online, free of charge, as soon as it has been published. One alternative is "green" open access, where a journal will make a paper freely available after a specified embargo period.

Since 2006, the Wellcome Trust has required the researchers it funds to make their results available for free, through the UK PubMed Central repository, within six months of initial publication. It even makes funds available to its researchers to pay for publication costs in academic journals, thus allowing researchers to choose gold or green open access models. Despite the stipulation, though, only 55% of trust-funded research papers are open access.

"We are firmly committed to ensuring that research publications that result from our funding are made freely available to all," said Walport. "Yet, despite our open access policy having been in place for over five years, still almost half of these publications remain restricted behind subscription paywalls. This is simply unacceptable and so with immediate effect we will be tightening up enforcement of our policy."

In a letter sent on Wednesday to university vice-chancellors and the trust's grant holders, Walport outlined the planned sanctions against scientists who do not follow the open access policy.

"When trust-funded researchers prepare final grant reports, we will require the principal investigator's head of department to provide signed assurance that all papers associated with the grant comply with the trust's policy. If they are unable to do this, the final payment on the grant will be withheld," said the letter. This is typically 10% of the full grant.

"That's an incentive for the institution to put pressure on the principal investigators that all the papers that came out of the grant are open access," said Robert Kiley, head of digital services at the Wellcome Trust.

From 2013, the open access conditions will mean that a paper resulting from Wellcome Trust grants should be freely available for any use, including commercial, providing the source is acknowledged.

Future applications for research money or renewal of grants will also be affected. Any Wellcome-funded work that has not been made open access will be discounted from a researcher's publication record when they make their grant application. The conditions apply to any research papers published after 1 October 2009.

"It's all about changing behaviour," said Kiley. "Getting researchers to accept that, if they're going to take Wellcome Trust money, then open access is not just an option, it's a requirement."

The UK government has already signalled its support for open access to scientific research funded by taxpayers and has drafted in Wikipedia founder Jimmy Wales to help achieve its goal. Last week, a study commissioned by universities and the science minister, David Willetts, also concluded that open access was the future for academic publishing.

"I am encouraged by the Wellcome Trust's continued commitment to expanded access and our shared recognition that its provision should be a core cost of the conduct of research," said Willetts. "Opening up access to research findings is a key commitment for this government. It will drive growth, foster innovation and usher in a new era of academic discovery and collaboration. But we also recognise the importance of the peer review process and the value added by academic publishers."