The prime minister's pledge to put shareholders at the forefront of the battle on executive pay may not be enough to end soaring bonuses, he was warned as it emerged that just 18 of the company remuneration policies put to a vote since Labour first gave investors a say on pay nearly 10 years ago have been defeated.

As David Cameron set out proposals to give shareholders a binding vote on executive pay deals, data compiled by a leading advisory body, Pensions & Investment Research Consultants (Pirc), showed how few remuneration votes had more than 50% of votes cast against them since investors were granted an advisory vote in 2003.

"We would be in favour of exploring a binding vote on pay, but the problem at the moment is that too few shareholders are willing to use the rights they have, as demonstrated by the low number of remuneration reports that have been voted down," a Pirc spokesman said.

Increasing numbers of shares in companies listed on the stock market are no longer owned by traditional institutional investors – such as pension funds – but by hedge funds and other overseas investors, which makes it more difficult to galvanise rebellions against pay deals.

Sarah Wilson, chief executive of the proxy voting agency Manifest, pointed out that in addition to outright defeats for pay deals there were large dissenting votes of more than 30% at companies such as WPP, Aberdeen Asset Management, Mitchells & Butlers, WH Smith and British American Tobacco. Such levels of rebellion are high as traditionally votes at annual meetings were usually in the region of 99% in favour of all resolutions.

Cameron signalled that Vince Cable, the business secretary – whose department will be holding round table discussions with investors next week – was also planning to make it easier to understand what directors were earning and change the mix of non-executive directors on company boards who currently set directors' pay. Speaking at the start of the annual bonus season in the City – beginning with big US banks this week – Cameron indicated that legislative changes could be announced in the spring, following a formal statement by Cable at the end of this month.

Bonuses are particularly sensitive at the bailed-out banks. Royal Bank of Scotland said no decisions had yet been made on any payments to its top staff, including chief executive Stephen Hester.

A pledge to clamp down on boardroom pay has been rising up the political agenda, prompting the TUC general secretary, Brendan Barber, to say that "all three main political parties are finally coming round to policies that the TUC has advocated for a decade."

The government asked in a recent consultation whether a ratio of boardroom pay to average employees should be published – although Cameron appeared to be backing down from this.

Lord Myners, former City minister, said shareholders were not making a great enough impact. "The idea of publishing ratios and getting an employee on the board are noise around the edges as the real lacuna is the failure of the owners to take an active interest," said Myners.

Deborah Hargreaves, chairman of the High Pay Commission – set up by the leftwing pressure group Compass and backed by money from Joseph Rowntree Charitable Trust – said it could not all be left to shareholders. "We welcome the prime minister's words but you can't rely on shareholders to solve it," she said.

Questions were asked whether a binding vote – supported by the Institute of Directors – would solve the issue. John Cridland, CBI director general said it would be "shutting the stable door after the horse has bolted, as shareholders would only be voting after the problem has happened".

Cameron suggested that votes could be specifically on pay-offs to directors, which was welcomed by some City sources.

One of the main shareholder groups, the Association of British Insurers, said it was working with the government. Otto Thoresen, ABI director general, said: "UK institutional investors share the government's determination to strengthen shareholder rights, tackle excessive executive remuneration and improve the accountability and transparency of the companies in which we invest."

Wilson also pointed out that the issue of changing the mix of non-executive directors who sit on company boards may not solve the problem that Cameron described as "crony capitalism," where executives from rival companies end up setting each others' pay. According to her research only 5% of executive directors sit on the board of another FTSE 100 company as a non-executive director.

"It is all to easy to legislate for yesterday's problems. As the non-binding vote on remuneration policies showed, getting it wrong does more harm than good. Shareholders need protection from reward for failure and must be able to claw back undeserved pay awards," Wilson said.