The letter comes following a controversial column by Mr Johnson in the Daily Telegraph where he compared Muslim women wearing niqabs to letterboxes and bank robbers.
Sputnik spoke to John Curtice, professor of politics at Strathclyde University, on the issue.
Sputnik: What ramifications could the inquiry into his comments have for Mr Johnson's political career considering the fact that some view him as the biggest rival to Prime Minister Theresa May's weak leadership?
So, he was, in many respects, defending the position of women, but at the same time he was using the language to indicate that it wasn't something that he personally was particularly comfortable with, and he typically used rather colorful language. The second reason was much more directly political, and that is that it's pretty clear that those inside the Conservative Party who are willing to defend Mr. Johnson are essentially those who are unhappy with the Chequers agreement that the Prime Minister has come forward so far as what the UK's relations with the EU should be.
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These are primarily the people who are most critical. Mr. Johnson, of course, resigned from the UK government as a result of his own particular dislike of that agreement. I think that the honest truth is that Theresa May cannot afford an internally signed foreign war about Mr. Johnson's position because it wouldn't come in truth.
It would probably light the touch paper to what is already considerable discontent inside the Conservative Party about her strategy of the European Union. They actually cannot afford to ignite that particular touch paper. I think basically at the end of the day Mr. Johnson might be reprimanded in some way or another but I think that in the end this is not going to go that far because the Prime Minister is not in a position to let it go too far.
Sputnik: Let's talk about the possible implications for the already-divided Tory Party. As you mentioned there could be a problem for Theresa May if she were to further ignite [that touch paper]; how is this really affecting the party and has there really seen a further division of the Tories?
Given Mr. Johnson's personal position, it's going [to stay] remarkable for a week; it's very difficult to think of any previous newspaper columnist who would still be dominating the headlines a weak later.
Why is it? Frankly, this is not because of the content of the article, it's because this is the surrogate for a broader argument about the negotiations. At the moment, the EU negotiations themselves are going on the official level and therefore there isn't that much for the politicians to argue about how the negotiations are proceeding at the moment, but they do have Mr. Johnson's article to argue about and that's why this debate is continuing.
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In the end, it's an argument to those people who want a hard Brexit, who are concerned about the immigration and the UK's sovereignty; and this argument is simply feeding into this internal debate inside the Conservative Party.
Sputnik: Let's go back to the actual comments themselves. We know that there are some European countries that already have burqa laws, France for instance, but there's no special legislation in the UK yet. Can you talk about the public opinion surrounding this issue?
Certainly, this is formal clothing that only a minority of Muslim women wear, and Muslim women are still a distinct minority in the UK; but there is no doubt that for some people this is something about which they feel uncomfortable, these are people from the more socially conservative area, that's one end of the spectrum.
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The other end of the spectrum is that there're some people, and Mr. Johnson indicated this in his comments, who have some sympathy with this. Perhaps, they feel that Muslim women who are covering their faces aren't necessarily doing so out of free choice but they're rather doing so because their male partners feel that that's what they should do. And they think that this is a form of male oppression of women.
The views and opinions expressed by the speaker do not necessarily reflect those of Sputnik.