Conservative MPs have called Boris Johnson’s position “untenable” after the prime minister was fined by the Metropolitan police for breaching lockdown rules, while cabinet ministers have rallied to defend Johnson. Nigel Mills, the MP for Amber Valley, became the latest to call for the prime minister to resign, though several other critics of Johnson have said they will not push for a vote of no confidence because of the Russian invasion of Ukraine. In a letter to a constituent seen by the Guardian, Mills said: “I don’t think Boris Johnson’s position as prime minister is tenable after breaking the rules his government put in place. Those leading the country must have the highest standards, and therefore, as a result of these fines, I believe his position to be prime minister has become untenable. “I know many of my constituents are rightly angry that at a time when they were observing the very strictest of the rules, some in Downing Street didn’t have the decency to observe them.” Caroline Nokes, the former cabinet minister, confirmed to a constituent that she had already submitted a letter of no confidence in the prime minister. “I have already been very clear that I believe the PM’s conduct fell far short of what my constituents have every right to expect,” she wrote. “I do not need to write a letter of no confidence to the chair of the 1922 Committee, mine was in a very long time ago.” Another Conservative MP, Gary Streeter, who has submitted a letter of no confidence, said his position was “unchanged”. However, many of those who had previously submitted letters of no confidence – Sir Roger Gale, Andrew Bridgen and Douglas Ross – said now was not the time to change leader given the instability caused by Russia’s invasion of Ukraine. Overnight, cabinet ministers moved to back Johnson. The transport secretary, Grant Shapps, said Johnson was “mortified” that he had broken the rules by having a birthday gathering in the cabinet room. The chancellor, Rishi Sunak, was also given a fixed-penalty notice by police for attending the gathering. Shapps said the prime minister was sorry but went on to minimise the event by saying Johnson had just “walked into a room” where people were gathered to wish him a happy birthday for 10 minutes. Shapps insisted Johnson acted “without malice” and did not “knowingly break laws”. When asked how it had not occurred to the prime minister that he was breaking the rules, Shapps told BBC Breakfast: “I don’t seek to say anything other than it’s deeply disappointing. Of course, it should have done. “I think in answer to your question, specifically, he will have already been in that room with the same people that day chairing crisis meetings on the world’s biggest pandemic for over 100 years. “And he walks in by surprise and into the same room to the people who wished him happy birthday on this occasion. And that’s, of course, the thing which breached the law, and the police have ruled on that and he has paid the fixed-penalty notice of £50 and apologised.” Johnson and Sunak are believed to be the first sitting prime minister and chancellor to have criminal sanctions imposed on them. Both senior politicians were also accused of misleading parliament by previously denying they had attended parties during lockdown. Keir Starmer, the Labour leader, called on both the “guilty men”, Johnson and Sunak, to resign for “dishonouring their offices”. He said Johnson had “lied repeatedly to the public” and his behaviour was a “slap in the face” to all those who had followed the rules. “They have to go,” he said. Liz Truss, the foreign secretary and a potential future leadership contender, issued a statement supportive of Johnson, saying he had “apologised and taken responsibility for what happened in Downing Street”. She said: “He and the chancellor are delivering for Britain on many fronts, including on the international security crisis we face. They have my 100% backing.” Some Conservative MPs previously suggested a line would be crossed by allowing a prime minister to remain in office if he was found to have broken the law. About 20 to 30 MPs were at one point thought to have submitted letters of no confidence in Johnson, with 54 needed to trigger a leadership challenge.