Speaking on BBC Two’s Daily Politics, Jonathan Bartley accused the Prime Minister of “falling over herself” to meet the US President “cap in hand”.
He said Mrs May should have “bided her time” and gone with “clear lines” over issues such as torture after Mr Trump advocated waterboarding this week.
Responding to whether he thought Mrs May’s US visit was appropriate with some viewing the UK as “friendless”, he said: “It puts us in a very weak position – leaving the EU, we are the junior partner, we are looking quite desperate.
“And I think Theresa May looks quite desperate going over there, falling over herself to be the first in the queue.
GETTY•BBC TWO/DAILY POLITICS
Jonathan Bartley branded Theresa May "desperate" for meeting Donald Trump
I think Theresa May looks quite desperate going over there, falling over herself to be the first in the queue
Green Party co-leader Jonathan Bartley
“I think she should have bided her time and she should have gone with some very clear lines, particularly around torture and most importantly around the Paris climate deal.”
He then praised Mexico for standing up to the new President, who this week set in motion plans to build a wall at its border.
“Look at someone like Mexico, they’re showing us up at the moment, they’re being very very strong and they’re playing hardball,” said Mr Bartley.
“What we’ve got is a situation where we’ve put ourselves in an extreme Brexit context where we have to go cap in hand to America and say ‘what can you give us in terms of a trade deal’.
May meets Trump: Historic first meeting in pictures
Fri, January 27, 2017
The two leaders will spend about an hour in face-to-face talks in the Oval Office, where President Trump has restored a bust of Winston Churchill removed by predecessor Barack Obama.
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Prime Minister Theresa May speaks during the 2017
BBC TWO/DAILY POLITICS
Jonathan Bartley said Theresa May was "falling over herself" to meet the President
“Of course Donald Trump is welcoming her with open arms – he knows he’s got all the cards in his hand. He sees a very desperate Prime Minister saying ‘what can you give us please’.”
Tory MP Nigel Evans also appeared on the show, but praised Mr Trump and Mrs May for arranging to meet on Friday.
He said: “We’ve got in President Trump a man whose mum was Scottish… he was in favour of Brexit, he wants to do a trade deal with the United Kingdom as quickly as possible, he put the bust of Winston Churchill back in the oval office as one of his first acts.
“What is there that now we need to do is engage with Donald Trump as much as we possibly can and of course, Theresa is the first world leader to meet Donald Trump since becoming president.”