LONDON — Senior EU figures are telling Britain to forget about negotiating a trade deal with the US or any other state as long as it remains a member of the 28-nation bloc.

Prime Minister Theresa may is set to visit Washington this week where she will reportedly meet with President Donald Trump to discuss the early stages of a new trade deal between the two countries.

A Downing Street spokesperson said over the weekend that the pair would "discuss how we can deepen our already huge economic and commercial relationship to the benefit of both of our countries, including our shared ambition to sign a UK-US trade deal once the UK has left the EU."

The government also confirmed that before meeting the new President, May will meet with senior Republican congressmen whose backing will be crucial if Britain is to strike a deal with the US during the "America first" administration of Trump, the Times reports.

However, the UK government's eagerness to build new trade relations with Trump's US is irritating leading figures in Brussels, who are keen to remind May of her obligations as a leader of an EU member state.

Under current EU treaties, Britain can neither strike nor even begin to negotiate any trade agreement with countries outside of the Union until it has formally terminated its membership, which is expected to occur in March 2019.

This was made clear by the EU's foreign affairs commissioner Federica Mogherini, who used a meeting with UK Foreign Secretary Boris Johnson and his European counterparts to issue a stern reminder to Britain.

"It’s absolutely clear on the EU side that as long as a country is a member state of the EU, which is something the UK is at the moment, there are no negotiations bilaterally on any trade agreement with third parties. This is in the treaties and this is valid for all member states as long as they remain member states, until the very last day.

"This is in the treaties and this is valid for all member states as long as they remain member states, until the very last day," Mogherini told media after the meeting.

This is an obligation that EU leaders have made clear to the UK on numerous occasions in the past, but warnings issued from Brussels don't appear to be deterring May's government as it prepares for life as a non-EU member.

May will become the first foreign leader to hold talks with the Republican President since his inauguration last week.

The two sides are keen to explore a trade deal based on slashing export tariffs and making it less difficult for US citizens to work in the UK and vice-versa, according to multiple reports.

The thinking in Downing Street is that this sort of deal would allow Trump to stick to his "America first" promise while agreeing to a trade arrangement that benefits both sides. Sources inside the civil service say the current aim is to agree on "zero tariffs" for certain items exchanged between the UK and the US.

Speaking to Andrew Marr on Sunday, May said: "There will be many issues for us to talk about, because obviously the special relationship between the UK and the US has been strong for many years.

"We’ll have an opportunity to talk about our possible future trading relationship, but also some of the world’s challenges that we will face, issues like defeating terrorism, the conflict in Syria."