The government temporarily increased the benefit to help people through the Covid crisis and is worth £1,000 a year.
But the uplift is due to expire in April, potentially hitting the incomes of six million families.
Labour forced a non-binding vote in the Commons on the issue on Monday evening demanding the extra £20 per-week remain in place.
The motion was approved by 278 votes to zero, majority 278.
Despite Johnson instructing Conservative MPs to abstain, six Tories defied the government’s order and voted in favour of Labour’s motion. They were: Peter Aldous (Waveney), Stephen Crabb (Preseli Pembrokeshire), Robert Halfon (Harlow), Jason McCartney (Colne Valley), Anne Marie Morris (Newton Abbot) and Matthew Offord (Hendon).
Keir Sarmer, the Labour leader, said cutting the money to people who “desperately need it” in the middle of a pandemic was “the wrong thing to do”.
The prime minister told Tory MPs to abstain rather than actively vote against it, a move Starmer branded “pathetic”.
Former work and pensions secretary Crabb, rebelling for the first time against the party whip, said the weekly rise should be kept for a further 12 months in order to give people “certainty” over their finances.
Downing Street insisted no decision has been made on whether to keep or scrap the increase, and said Chancellor Rishi Sunak will update the public on the government’s plans “shortly”.
Johnson repeatedly declined to state whether or not the increase will be extended when questioned during a visit to Oxfordshire.
“What we have said is we will put our arms around the whole of the country throughout the pandemic,” the prime minister told reporters.
“We have already done £280 billion worth of support and we will keep all measures under constant review.”
He added: “It’s the policy of the opposition to abolish Universal Credit altogether, which I don’t think is a sensible way forward.”
The prime minister was also facing pressure from the 65 Conservative MPs in the Northern Research Group (NRG), who said ending the increase would be “devastating”.
Crabb praised Sunak’s “historic” financial support policies during the pandemic but told the Commons: “The question for us right now is whether at the end of March this year, just 10 weeks away, it’s the right time to begin unwinding this support – specifically to remove the extra support for Universal Credit claimants – and I don’t believe it is the right moment.”
Crabb said the extra £20 a week has helped those “right at the bottom of the income scale”, adding: “The truth is the labour market is a horrible place right now for many people.
“Opportunities for people to find new work, increase their hours, boost their earnings, improve their family finances have been massively curtailed by the economic impact of the public health emergency, and that’s the context for this discussion about cutting back the £20 per week uplift.
“It’s why I believe the uplift is so important right now and it’s why I believe it needs to be extended for a further 12 months.”
For the government, work and pensions minister Will Quince said it is right to wait for more clarity on the national economic picture before assessing the best way to support low-income families.
He said: “I have sympathy with the argument that it would give claimants certainty. However, one of the evident features of a pandemic is uncertainty.
“If (shadow work and pensions secretary Jonathan Reynolds) is certain about what the economic and social picture will look like in April, well, to be frank, he must have a crystal ball.
“The reality is we simply do not know what the landscape will look like and that is why it’s right that we wait for more clarity on the national economic and social picture before assessing the best way to support low-income families moving forward.”
Vaccines minister Nadhim Zahawi earlier derided Labour’s symbolic move as a “political stunt” and Johnson told Conservatives in a WhatsApp message to miss the vote as he accused Labour of “playing politics” with “legislatively vacuous opposition debates”.
The government temporarily increased the benefit to help families through the Covid crisis but the uplift is due to expire in April, potentially hitting the incomes of six million families.
The Resolution Foundation warned that scrapping the £20-a-week uplift will lead to a particularly tough 2021 for low-income households, whose incomes could fall by 4%.
The think tank estimated that the withdrawal of the benefit increase would drive up relative poverty from 21% to 23% by 2024-25, pushing a further 730,000 children into poverty.