A row has broken out over Boris Johnson’s hopes for a “big bang” reopening of schools, as sources claimed it had run into resistance from Prof Chris Whitty. The chief medical officer for England was said to be reluctant to put his name to a public show of support for the policy this week. Education sources had told the Guardian that Whitty was “very unhappy” with the idea of all 10 million children and staff returning to schools in England on 8 March, although the government denied this and insisted that Whitty was not opposed to any of the options being discussed. On Monday the prime minister is to announce the government’s roadmap for lifting national lockdown restrictions in place since the start of the year. While publicly ministers have committed to reopening schools “from” 8 March rather than all pupils returning on that date, No 10 is said to be planning for an across-the-board return for all year groups. But school leaders are increasingly worried, having expected a phased return, especially in secondary schools, to enable repeat Covid tests to be administered. Schools are currently open only to vulnerable pupils and the children of key workers. Ministers and senior policymakers, including permanent secretaries, have attempted to convince Whitty that he needs to make a public statement of support for the policy to reassure parents and teachers about safety, it is understood. But so far he has not provided an endorsement to be included in the media materials to be distributed ahead of Monday’s announcement, sources said. Instead Whitty has offered support described as “lukewarm” and is expected to say it is down to politicians to make the final call on the timing of school reopenings. An education source said they believed the row could be resolved, adding: “No 10 will come up with a formulation of words that Whitty can live with.” A government spokesperson said the claims were “categorically untrue”, adding: “We know schools, parents and pupils need clarity on plans as soon as possible, which is why we have committed to providing two weeks’ notice for them to prepare … Pupils will return from 8 March at the earliest.” A No 10 source said Sage advice on the roadmap would be published “after the event”. While the spread of the virus in schools is still being studied, interim data from the React-1 study this week revealed that while Covid infections had fallen by two-thirds, they were most prevalent among those aged five to 12 and 18 to 24. Researchers suggested this could be due to a significant proportion of primary pupils still in classrooms during lockdown. Like last year’s plan for lifting lockdown, Johnson’s roadmap on Monday is expected to outline the earliest dates when various sectors of the economy could be reopened if coronavirus data allows – with a warning that these could slip if the situation deteriorates. “If we need to move things to the right because of our assessment of the pandemic, then we may have to do that,” said a Downing Street source. The prime minister has repeatedly stressed that schools will be the first things to reopen, with non-essential retail expected to follow, and hospitality after that. Ministers are also eager to build in a pause after children return to the classroom to allow the impact on the spread of the virus to be assessed. However, there is unlikely to be much solid good news for families hoping to book a summer break abroad in the document accompanying the announcement, with Johnson and the health secretary, Matt Hancock, still keen to monitor the emergence of variants before lifting curbs on foreign travel. The chancellor, Rishi Sunak, will follow the roadmap with a budget on 3 March that is expected to extend business support measures due to run out at the end of the month. Government sources said Sunak is preparing finally to announce a temporary extension of the £20-a-week uplift in universal credit at the budget. Anti-poverty campaigners including the footballer Marcus Rashford have called for the boost to be extended but Sunak is reluctant to make it permanent, with a six-month extension likely. Johnson has been under intense pressure from his party to ensure all schools reopen as soon as possible, with concerns about the issue stretching well beyond the vocal Covid Recovery Group of backbenchers. Three MPs – education select committee chair Robert Halfon, former health minister Steve Brine and Lib Dem MP Munira Wilson – have scheduled a backbench debate on Thursday about the “Covid recovery plan” for schools, which could become a flashpoint for criticism of the prime minister if he is regarded as being too timid about reopening. Some colleagues believe Johnson has become “captured” by his cautious scientific advisers. The roadmap for England will be finalised on Sunday at a meeting of the government’s Covid strategy committee. The full cabinet will then rubberstamp it, probably on Monday morning, before Johnson announces it in the Commons at about 3.30pm. Later he will chair a Downing Street press conference, where he is expected to be flanked by Whitty and chief scientific adviser Patrick Vallance. The pressure on Whitty to wholeheartedly back a “big bang” return for pupils comes as school leaders and unions issued a rare joint statement warning schools should not reopen unless the scientific evidence is unambiguous. The statement warned: “We are increasingly concerned that the government is minded to order a full return of all pupils on Monday 8 March in England. This would seem a reckless course of action. It could trigger another spike in Covid infections, prolong the disruption of education, and risk throwing away the hard-won progress made in suppressing the virus over the course of the latest lockdown.” The group said Johnson should “commit to 8 March only if the scientific evidence is absolutely clear that this is safe, and at that point go no further than a phased return of children and young people with sufficient time to assess the impact before moving to the next phase”. Saying school reopening would directly affect 10 million students and staff, nearly a fifth of England’s population, the group said: “This is not a small easing of lockdown restrictions. It is a massive step.” Signatories include the Association of School and College Leaders (ASCL) and National Association of Head Teachers as well as the main teaching unions, and Unite, the GMB and Unison representing school support staff. School governors, sixth form and further education college representatives also signed. A compromise on reopenings may allow primary pupils and secondary school pupils taking exams to return immediately, with other secondary students coming back as testing arrangements allow. Geoff Barton, general secretary of ASCL, has argued for schools to be given the ability to delay the return of year groups in order to carry out the testing required by the government. Those pupils that test negative would then be able to return to the classroom immediately. In Wales the first minister Mark Drakeford announced that primary schools would fully reopen on 15 March following children up to the age of seven returning from Monday. In Scotland children up to the age of seven will also return from Monday, as will some pupils taking exams. Other students are likely to return in mid-March.