LONDON (Reuters) - Prime Minister Theresa May will discuss Brexit and a bilateral trade deal with U.S. President Donald Trump on Wednesday on the sidelines of the United Nations General Assembly in New York, amid increasing uncertainty over Britain’s EU exit plans.

FILE PHOTO - Britain's Prime Minister Theresa May arrives to make a statement on Brexit negotiations with the European Union at Number 10 Downing Street, London September 21, 2018 . Jack Taylor/Pool via Reuters

“She will be discussing a post-Brexit trade deal amongst a range of subjects also including chemical weapons and foreign affairs,” a senior UK official said on Monday.

“When they meet they often discuss Brexit negotiations and they both have a strong commitment to securing a strong UK-U.S. free trade agreement once we’ve left (the European Union).”

A trade deal with the United States is seen as an important way to help offset the economic impact of leaving the EU in March next year. May’s government has long touted the freedom to strike such deals independently of the EU as the main economic benefit of Brexit.

Any supportive comments from Trump could boost May’s struggle to win over critics who say her exit plan is unworkable, and soothe financial markets which expect severe disruption if she is unable to get a Brexit deal.

During a visit to Britain earlier this year Trump said the two countries could secure a “great” post-Brexit trade deal, lavishing praise on May and contradicting his own earlier withering assessment of her exit strategy.

The U.N. meeting is likely to be dominated by Trump touting his drive to protect U.S. sovereignty, which has some world leaders worried over America’s commitment to the multilateralism that has governed the United Nations since World War Two.

May will also use her two-day trip to New York to warn against allowing the use of chemical weapons to become normalised, citing the use of a nerve agent in Britain earlier this year and chemical attacks in Syria.

“The red lines around the use of chemical weapons are being eroded,” she said ahead of a meeting of the U.N. Security Council on Wednesday.

“The international community needs to do more together - both to prevent future chemical weapons use and to ensure those who use them are held to account, but also to tackle the range of other threats to global security, including the proliferation of WMD (Weapons of Mass Destruction).”

She will also use a speech at a business event to defend capitalism and free markets, and hold bilateral meetings with other leaders including Iranian President Hassan Rouhani.