Overnight, Trump announced new tariffs – or import taxes – of 25% on cars and car parts coming into the US from 2 April.
The UK has already been hit by 25% tariffs on steel and aluminium exported to the US.
Meanwhile, Trump has promised a raft of further tariffs from 2 April.
Reeves told the BBC the UK is in “intensive negotiations” with the US to carve out a deal to avoid tariffs.
However, the prospect of a global trade war still threatens to wipe out the headroom in the country’s finances.
Mr Johnson said Trump’s announcement overnight demonstrated “we live in a risky and changing world”.
“There is a good chance that economic and fiscal forecasts will deteriorate significantly between now and an autumn Budget,” he said.
“If so, she will need to come back for more; which will likely mean raising taxes even further.”
He added that “months of speculation” over which taxes might rise could have “damaging” political and economic consequences.
Asked on Wednesday if there could be more tax rises in October, Reeves said: “We’ll never have to do a Budget like that again.”
Pressed on BBC Radio 4’s Today programme over whether if things went wrong there may need to be more tax rises or spending cuts, the chancellor acknowledged “there’s always risks” but said there were also “opportunities” for economic growth from housebuilding and reforms to the planning system.
