The chancellor is a "gnats whisker" away from having to hike taxes further, the Institute for Fiscal Studies (IFS) has said.
Paul Johnson, the director of the independent economics think-tank, has given his verdict on the spending review.
He said the chancellor is "now going to have all her fingers and all her toes crossed, hoping that the OBR will not be downgrading their forecasts in the Autumn".
"With spending plans set, and 'ironclad' fiscal rules being met by gnat鈥檚 whisker, any move in the wrong direction will almost certainly spark more tax rises," he said.
Johnson added that Reeves has had to take "some incredibly tough decisions" and has made a "perfectly reasonable set of prioritisations".
But he warned: "The real test will be in how well the money is spent, and especially how effectively the capital spending is spent and managed.
"That's the tough day-to-day business of government and, somewhat, out of the spotlight of these big set piece events. It's on that more than anything that the government will be judged."
IFS left 'baffled' by Reeves' speech which was 'not serious'
Speaking of Reeves, Johnson will be fairly critical about appearance in the Commons yesterday.
He said the IFS was left "baffled by the chancellor's speech" because it did "not appear to be a serious effort to provide any useful information to anybody".
Johnson said Reeves has made some "rather odd recent claims" about the economy improving.
But he added that "neither the economic forecasts nor the public finances have improved relative to... a year ago".
"Rather, the reverse."