(Corrects headline to third quarter, not year-to-date; in third- to-last paragraph adds dropped words)

By Greg Roumeliotis and Pamela Barbaglia

NEW YORK/LONDON, Sept 28 (Reuters) - The value of mergers and acquisitions globally dropped slightly in the third quarter of 2017, as big deals worth more than $10 billion were scarce given uncertainty about economic policy in the U.S. and Europe in particular, leaving dealmakers to feast on a plethora of smaller transactions.

Even as major stock markets continued to climb higher, big companies were wary of pursuing transformative deals in the quarter, as the future of U.S. President Donald Trump's agenda on taxes, healthcare and infrastructure spending remained unclear, while Britain's Brexit talks, and North Korean's nuclear ambitions also weighed on chief executives' appetite to take risks.

"Jumbo deals have subsided in part because of the continued uncertainty over tax policy and deregulation - removing that overhang would be a positive catalyst for M&A," said Matt McClure, Americas head of mergers & acquisitions at Goldman Sachs Group Inc. "Even if it becomes clearer that the status quo isn't going to change soon, you may see companies revisit larger transactions that they have put on hold."

The value of global merger and acquisitions slipped to $765 billion in the third quarter, down 5 percent year-on-year and the lowest third-quarter level since 2013, according to preliminary Thomson Reuters data.

"It's a big bet to pursue a mega-deal in this environment and boards need more time to act. For sub-$5 billion deals, instead, there are fewer hurdles and it's easier to get to the finish line," said Steven Baronoff, chairman of global M&A at Bank of America Corp.

The biggest deal to be signed in the third quarter was the U.S. aerospace and industrial company United Technologies Corp's $30 billion cash-and-stock acquisition of U.S. avionics maker Rockwell Collins Inc.

"After the summer break there has been no big rush to get‎ the ball rolling on deals. This could be a sign that activity will remain flat or subdued and political uncertainty will continue holding back large transactions," said Scott Hopkins, an M&A Partner at law firm Skadden, Arps, Slate, Meagher & Flom LLP in London.

Third-quarter M&A volume fell to $309 billion in the United States, down 6 percent year-on year. In Europe, M&A totaled $343 billion, down 15 percent year-on-year, while in Asia, M&A was 226 billion, up 11 percent year-on-year.

"The market for deals overall has held up well, given the rich valuations and macroeconomic and geopolitical uncertainty," said Mark Shafir, co-head of global M&A at Citigroup Inc.