These pressures are driven by high unemployment, macroeconomic imbalances and tensions over foreign exchange rates, said the heads of the World Trade Organization (WTO), Organisation for Economic Co-operation and Development (OECD) and United Nations Conference on Trade and Development (UNCTAD).
"The stability of the trading system will be put at considerable risk if currencies move in what some perceive as the pursuit of an exchange-rate-induced comparative advantage," they said in a summary of reports ordered by the G20 for its summit in Seoul next week.
"We urge G20 governments to address these risks," WTO Director-General Pascal Lamy, OECD Secretary-General Angel Gurria and UNCTAD Secretary-General Supachai Panitchpakdi said in the summary, whose message was reported by Reuters on Wednesday.
Shift in Tone
The G20 committed to keeping markets open at its first summit in Washington two years ago to deal with the financial crisis and also at subsequent meetings. It commissioned regular reports from the three organisations to monitor trade and investment policy for protectionism.
Previous reports from the three had concluded that protectionism was being held in check, so the warning marks a significant shift in tone.
The WTO forecasts that global export volumes will rebound by an unprecedented 13.5 percent this year. But trade, both a motor and reflection of recovery, has slowed in recent months and is at risk from economic uncertainty and protectionism.
WTO chief Lamy and UNCTAD officials have warned in recent weeks specifically of the risks to the economy of currency tension, but the direct warning from the three heads to the G20 in the report shows just how strong these concerns have become.
Tensions over exchange rates and monetary policy have bedevilled U.S.-Chinese relations for months and many other countries, including G20-host South Korea, Japan and Brazil have also sounded the alarm about their partners' policies.
The Federal Reserve launched a new effort to support the U.S. economy by printing money to buy bonds, but critics fear the policy will lead to high inflation and worry low interest rates in the United States could fuel asset bubbles in other countries and destabilise currencies.
In a report on trade, the WTO said G20 countries had continued to exercise restraint in imposing new restrictions since their last summit in Toronto at the end of June.
New measures, increasing but at a slower rate, covered 0.3 percent of G20 imports and 0.2 percent of total world imports.
But the steady accumulation of measures since the financial crisis burst in 2008 meant restrictions now covered 1.8 percent of G20 imports, and only 15 percent of them had been withdrawn.
"This is too low. G20 governments need to give priority to removing those measures," the three heads said.
On foreign investment, UNCTAD and the OECD said the G20 countries were resisting protectionism, with the majority of measures taken by 17 G20 states since the last summit aiming to facilitate and encourage investment flows.
But James Zhan, who heads UNCTAD's investment and enterprise division, said this assessment did not include the protectionist way in which governments were handling existing rules.
"We do observe a kind of covert investment protectionism in the implementation of existing investment policies," he told a news conference. He noted approval of new investment projects was particularly subject to obstacles -- an apparent reference to Canada's decision to block BHP Billiton's $39 billion bid for Potash Corp.
The share of restrictive measures in total investment policies had risen to 30 percent in 2009 from 2 percent in 2008.
Flows to G20 countries of foreign direct investment such as cross-border mergers and greenfield investments plunged 36 percent in the second quarter of this
