Image caption Dave Penman (right) said Ross Thomson's defence of Boris Johnson over the ambassador's resignation was "humiliating"

The head of a civil service union has criticised a Scottish Tory MP over comments he made about the resignation of the UK's ambassador to the US.

Ross Thomson, who is Boris Johnson's Scottish campaign chairman, said the "game was up" for Sir Kim Darroch when President Trump publicly attacked him.

Mr Thomson said the "national interest" should come before defending diplomats.

But Dave Penman, head of the FDA union, which represents senior civil servants, said this response was "humiliating".

Other politicians, including Scottish Conservative leader Ruth Davidson, have defended Sir Kim, saying "the diplomatic corps have to be protected and defended".

Sir Kim resigned as the UK's ambassador to the United States on Wednesday after a row over leaked emails in which he described the Trump administration as "dysfunctional", "unpredictable", "faction-riven" and "inept".

This prompted President Trump to tweet that the "wacky" ambassador was "a very stupid guy" whom he would no longer deal with.

Sir Kim said the situation was "making it impossible for me to carry out my role as I would like".

Some Whitehall sources have claimed that he decided to resign after seeing Mr Johnson - the frontrunner in the Conservative leadership contest - refuse to support him during a television debate on Tuesday evening.

An investigation into how the confidential messages were leaked to the Mail on Sunday newspaper is ongoing.

Image copyright EPA Image caption Sir Kim Darroch (right) pictured with then-Foreign Secretary Boris Johnson in 2017

Many leading politicians voiced dismay at Sir Kim's resignation, with both Mrs May and Labour leader Jeremy Corbyn stating their disappointment in the immediate aftermath.

Scottish First Minister Nicola Sturgeon said it was "shameful that Kim Darroch has effectively been forced out for doing the job that diplomats are appointed to do".

She added: "Boris Johnson's failure to stand up for him - and stand up to the behaviour of Donald Trump - spoke volumes."

Ms Davidson said: "It is the job of diplomats to report accurately and give their professional analysis honestly. Leaks should not happen.

"In the (very) rare occasion where they do, the diplomatic corps have to be protected and defended, or we degrade the whole service."

However Mr Thomson disagreed, saying that "the game was up" for Sir Kim when President Trump said he could no longer work with him, adding that "I don't think you defend diplomats when it is against the national interest".

He said: "We have to be realistic about this, the president of the United States himself has said I cannot work with this person. We have to make sure we have the best possible relationship with the US, and that means having someone that the US administration can trust.

"I know I'm maybe going against the flow of wider opinion, but I think Boris Johnson has shown leadership by not just playing to the crowd and saying what everyone else has been saying.

"I think it's really important that in the national interest we have the best possible relationship with the United States. I think it's really important post-Brexit in terms of new trade deals, in terms of security and defence.

"We actually had the president saying that he could no longer with that ambassador, given the unvarnished comments that were released, and essentially when that happened it was right to move on."

In response to this, Mr Penman - general secretary of the FDA union - said that "in a crowded field of MPs prepared to humiliate themselves to prove their unwavering support for Boris, Ross Thomson's definition of 'true leadership' is the clear winner".

Mr Penman said there had been a "clear" message to civil servants that "you are expendable", arguing that "the civil service needs protection like never before".

He also slammed the original leak of the emails, telling the BBC's Newsnight programme it was a "deliberate plot" which had "damaged our country" and created "tension about whether civil servants can trust ministers to have their backs".