The growth of internet access around the world has slowed dramatically, according to new data, suggesting the digital revolution will remain a distant dream for billions of the poorest and most isolated people on the planet.

The striking trend, described in an unpublished report shared with the Guardian, shows the rate at which the world is getting online has fallen sharply since 2015, with women and the rural poor substantially excluded from education, business and other opportunities the internet can provide.

The slowdown is described in an analysis of UN data that will be published next month by the Web Foundation, an organisation set up by the inventor of the world wide web, Sir Tim Berners-Lee. The data shows that growth in global internet access dropped from 19% in 2007 to less than 6% last year.

“We underestimated the slowdown and the growth rate is now really worrying ,” said Dhanaraj Thakur, research director at the Web Foundation. “The problem with having some people online and others not is that you increase the existing inequalities. If you’re not part of it, you tend to lose out.”

In 2014 the UN predicted that half the world would be online by 2017, but the slowdown means that line will not be crossed until May 2019, only months before the UN sustainable development goal of affordable internet access for all by 2020. The UN defines being online as having used the internet from any device in any location at least once in the past three months.

Had growth rates held steady near the 11% average for 2005 to 2017, more than half a billion extra people would now be online. Of the 3.8 billion who remain unconnected, an alarming proportion are women. In poor urban areas, men can outnumber women on the internet as much as two to one.

Beyond missing out on economic opportunities, people who are unconnected are cut off from online public debates, education, social groups and the means to access digital government services such as filing taxes and applying for ID cards. “As our daily lives become increasingly digital, these offline populations will continue to be pushed farther to the margins of society,” the report states.

Malcolm Johnson, deputy secretary general of the UN’s International Telecommunication Union, said 2018 data to be released in December is expected to show the slowdown continuing. “Something different has to happen to change the trend,” he said. “We need much cheaper connectivity and there has to be more work done on content to attract people.”

A number of studies have found evidence that internet access drives economic growth. A 2012 report from the University of California, Berkeley, linked a 10% rise in broadband access to a 1.35% increase in GDP in developing countries. A Deloitte report the same year claimed that a doubling in mobile data use boosted GDP per capita by half a percentage point.

Other benefits appear to be borne out on the ground. An October survey by the Pew Research Center in Washington DC found that the public in six sub-Saharan countries saw widespread advantages in being online. Across Ghana, Kenya, Nigeria, Senegal, South Africa and Tanzania, 79% felt that increasing internet access improved education, and more than half believed it benefited the economy, politics and even personal relationships. There was less agreement on morality: only 45% believed internet access had a positive impact.

Many of those offline are in areas that are difficult, and therefore costly, to hook up to the internet. The expense puts telecoms providers off because the communities are those least able to afford the high prices they must charge to get a return on the investment. At the same time, the internet may have little appeal for people in the world’s most remote regions. Even if they can afford the mobile phone and data costs, they may lack the skills to go online, and find little of interest in a language they know if they do.

“It’s not just about connectivity,” said Johnson. “You have to make it worthwhile for people to pay to connect. There has to be content they can understand and is of benefit to them.” He said efforts to provide internet access through new satellite constellations or high-altitude balloons could make a “huge difference”.

The persistent wage gap between men and women plays a large part in the digital gender divide but is far from the only factor. “Women are more likely to be left out because of economic inequalities and to a great extent social norms,” said Nanjira Sambuli, who leads the Web Foundation’s efforts to promote equal access to the web. “In some communities the whole idea of women owning anything of their own, even a mobile phone, is frowned upon.”

She added: “It’s a stark reminder that technology is not a silver bullet that is going to solve inequalities that exist and have continued to exist because of real factors that need to be addressed. These are challenges that have been kicked down the road.”