TOKYO (REUTERS) - Asian stocks fell the most in three weeks on Wednesday (Sept 25) after US lawmakers called for an impeachment inquiry into President Donald Trump, increasing the prospects of prolonged political uncertainty in the world's largest economy.

The dramatic move by Democrats in the House of Representatives compounded investors' anxieties with confidence already shaken by US-China trade tensions and global recession risks.

MSCI's broadest index of Asia-Pacific shares outside Japan fell 0.86 per cent, the biggest decline since Sept. 3, while Tokyo's Nikkei slipped 0.39 per cent for its largest loss in three weeks.

Chinese shares slumped and the offshore yuan fell, while oil futures extended declines after Trump harshly criticised Beijing's trade practices in a speech at the United Nations, damping hopes for a resolution to the US-China trade war.

Chinese shares fell 0.55 per cent while shares in Hong Kong skidded by 1.01 per cent.

Singapore stocks fell 0.78 per cent while Australian shares were down 0.54 per cent.

The downturn in Asia extended to Europe, with Euro Stoxx 50 futures off 0.20 per cent, those for Germany's DAX down 0.30 per cent and FTSE futures off 0.26 per cent.

"The impeachment probe has put a dent in Asian shares," said Kiyoshi Ishigane, chief fund manager at Mitsubishi UFJ Kokusai Asset Management Co in Tokyo.

"Chinese shares were already exposed to downside risks. Trump's comments likely increased those risks. There are worries about US consumer sentiment. There are also concerns that China's economic slowdown hasn't stopped."

The British pound fell against the US dollar, giving up some of its overnight gains, as persistent uncertainty over how Britain will complete its divorce from the European Union dogged the pound.

On Tuesday, the UK Supreme Court ruled that Boris Johnson's decision to shut down parliament in the run-up to Brexit was unlawful, a stinging judgment that undermined the prime minister's already fragile grip on power.

US stock futures rose 0.19 per cent in Asia on Wednesday, but those gains didn't translate into better sentiment for regional share markets.

The push for an impeachment inquiry and disappointing US economic data weighed on Wall Street on Tuesday, driving the S&P 500 0.84 per cent lower, its biggest daily decline in a month.

The US House of Representatives will launch a formal impeachment inquiry over whether Trump sought help from the Ukraine to smear former Vice President Joe Biden, a front-runner for the 2020 Democratic presidential nomination. Trump has denied the claims.

It is unlikely that the impeachment inquiry would lead to Trump's removal from office. Even if the Democratic-controlled House voted to impeach Trump, the Republican-majority Senate would have to take the next step of removing him from office after a trial.

However, the development injects fresh uncertainty into financial markets, which have already been roiled by political disquiet in Hong Kong to Britain to Italy and the Middle East.

"If an impeachment enquiry looks like ending his re-election chances in 2020, he may throw caution to the wind and harden his attitude to a China trade deal, increasing the chances of a global recession next year," said Jeffrey Halley, Senior Market Analyst, Asia Pacific, OANDA.

The US dollar index measuring the greenback against a basket of six major currencies was 0.20 per cent higher.

In the offshore market, the dollar rose 0.17 per cent to 7.1184 yuan.

Trump said on Tuesday Beijing had failed to keep promises it made when China joined the World Trade Organization in 2001 and was engaging in predatory practices that had cost millions of jobs in the United States and other countries.

US crude dipped 0.59 per cent to US$56.95 a barrel. Brent crude fell 0.82 per cent to US$62.58 per barrel.

The US-China trade war, which has dragged on for more than a year, has added pressure on China's already slowing economy and has increased the risk of recession in other countries. Slower global growth would hurt demand for energy and other commodities.

Sterling traded at US$1.2465, down 0.26 per cent, having trimmed some of its overnight gains made following the UK court ruling.

In response to the UK Supreme Court's decision, Johnson said he disagreed and vowed that Britain would leave the EU by Oct. 31 deadline, come what may.

Calls for Johnson's resignation could grow, which would cast even more uncertainty over how the UK would complete its divorce from the EU.

The yield on benchmark 10-year Treasury notes rose to 1.6576 per cent, while the two-year yield rose to 1.6216 per cent.