Theresa May is facing a powerful new cross-party campaign to derail her flagship education reform programme to expand the number of grammar schools in England.
Conservative former education secretary Nicky Morgan has joined forces with Liberal Democrat former deputy prime minister Nick Clegg and Labour ex-shadow education minister Lucy Powell to oppose the plan.
In a joint article in The Observer, they argue that creating new grammar schools will do nothing to promote social mobility and warn there is no room for more "division or political ideology" in the education system.
"We must rise to the challenge with a new national mission to boost education and social mobility for all," they write.
"That's why we are putting aside what we disagree on, to come together and to build a cross-party consensus in favour of what works for our children not what sounds good to politicians."
Their intervention is likely to set alarm bells ringing in Downing Street after other influential Conservatives, including the chairman of the Commons Education Committee Neil Carmichael, also voiced opposition to the plan.
With a working majority of just 17, Mrs May's vulnerability to Tory revolts was underlined last week when Chancellor Philip Hammond was humiliatingly forced to back down over his Budget reforms to National Insurance following a backlash from the backbenches.
In their article, the three say that an "endless debate" about more selection in the education system simply risked squeezing out positive developments that were taking place elsewhere.
"Those championing selection as the silver bullet for tackling social mobility, or as the panacea for creating good new school places, are misguided," they said.
"All the evidence is clear that grammar schools damage social mobility.
"Whilst they can boost attainment for the already highly gifted, they do nothing for the majority of children, who do not attend them. Indeed, in highly selective areas, children not in grammars do worse than their peers in non-selective areas.
"In a time when resources are so limited and many other educational reforms are still in their infancy or yet to be proven - from University Technical Colleges and new T-levels to the expansion of free childcare and hundreds of new free schools - now is not the time for more division or political ideology in education."