Firms are queuing up to list on equity indexes globally, sucking cash from existing listed stocks and fueling worries that these high flotation volumes could be signaling an impending market fall.

So far this year, the value of IPOs worldwide has soared close to levels last seen in the dying days of the tech bubble of 2000, supported by central bank policies that have pumped out cheap money and underpinned market gains over the past five years.

While high levels of IPOs can be healthy, feeding firms capital to expand their businesses, valuation ratios on a number of stocks entering the market have risen to levels that can signal the market is due for a sharp correction.

Investors have been dumping shares in recently floated firms, especially in the tech sector, as concerns mount about valuations, reviving memories of the dotcom crash.

These IPOs are also soaking up cash. Despite huge inflows into equities since the start of the year, the MSCI All-Country World Index, which tracks shares in 45 countries, is trading only slightly higher over the period.

"The new paper which is coming to market is diluting the technical support from fund flows... It does leave us vulnerable," said Ian Richards, global head of equities at Exane BNP Paribas.

"We know that there's still a big IPO pipeline ... Some of those deals will try to be squeezed through, then potentially that's a suppressant for markets."

Global-listed IPO volume, at $65.8 billion via 331 deals so far in 2014, represents the highest year-to-date value since 2010 and is just shy of a 2000 peak, data from Dealogic shows.

Meanwhile, equity funds globally have taken in more than $84 billion this year, according to EPFR Global data.

"Many times, what fund managers will do is that they will make sales in the secondary market to subscribe to those (initial public) offerings," said Ashish Misra, head of investment policy at Lloyds Bank Private Banking.

While the IPO stream remains robust, many of the companies listing have poor-quality earnings. The proportion of US firms coming to the market that are unprofitable is now at 74 percent, its highest since 1999, analysts at Redburn said.

Concerns about overstretched valuations have meant scant demand for new firms that might once have drawn huge interest. Chinese pork producer WH Group, for example, pulled its Hong Kong IPO after failing to get the valuation it wanted.