May, whose address to the Tory conference this week was wracked by cough, an imposter and letters falling off a bold slogan on stage, told The Sunday Times that she is "pretty resilient", and added that it was not her style to "hide from a challenge".
Claiming the Tories are "in all sorts of trouble" over Brexit, Ms Thornberry added: "It doesn't really matter who their leader is, they don't know where they are going".
Following the turmoil of the Conservative party conference in Manchester, Mrs May has faced repeated questions as to whether Mr Johnson was "unsackable" due to her weakened position.
"He has completely failed", a Cabinet minister told the Telegraph. "It seems blind to the facts, preoccupied with preserving "access" to the European Union market seemingly at any cost", he said.
Tory MP Nadine Dorries was even more direct, telling ITV's Peston on Sunday: 'If I were prime minister, the person I would be demoting or sacking would be Philip Hammond'.
She said: "I think Philip Hammond has been deliberately trying to make the Brexit negotiations hard, stall them, obfuscate the issues".
"I'm sorry to be so blunt but I think Boris Johnson as Foreign Secretary is not good for the United Kingdom".More news: Kaepernick says he'll stand during national anthem
Johnson's clarification has being backed by Crispin Blunt, who told Sky News today that the foreign secretary had supported May in his conference speech and now in the WhatsApp message: "He's declaring his loyalty, as he did in his speech, and he's declaring it privately".
"The country has had enough of the self-absorbed and, frankly, disloyal behaviour we have witnessed over recent weeks", he wrote, calling for the party to unite and for the government to "win back hearts and minds" or risk the prospect of "neo-Marxist" Corbyn taking the keys to Number 10.
Shapps said about 30 lawmakers want a new Conservative leader and prime minister.
In a message this morning to Conservative MPs Mr Johnson claimed he had no idea where the briefings came from and insisted they did not speak for him.
In an interview with the Sunday Times, May sent a strong signal about her intentions when asked specifically what she planned to do about her rebellious foreign secretary, the former London mayor who's in the past been touted as a potential successor.
Johnson, however, will "just say no" if May seeks to demote him, according to the Telegraph (paywall).
He replied: "The PM was talking broadly about how she is a politician who never ducks a challenge".