In an exclusive interview with HuffPost UK, Ellie Reeves said the Conservatives had stopped campaigning in Runcorn and Helsby.
Voters in in the seat will go to the polls on Thursday after the previous Labour MP, Mike Amesbury, quit after being convicted of punching a constituent.
Despite Labour holding the seat with a majority of nearly 15,000, bookies have made Reform UK odds-on favourites to win it in what would be a major blow for Keir Starmer.
With less than 72 hours to go until the polls open, minister without portfolio Reeves accused the Tories of leaving the way clear for Nigel Farage’s party.
She said: “The Tories aren’t doing anything. [Tory MP] Esther McVey basically said the Tories should sit it out and let Reform win, and we’re seeing that on the ground.
“They’re not doing any work on the ground at all, it looks like they’re just gifting it to Reform.”
Reeves’ comments will be seen by many as an admission that Labour are on course to lose the by-election.
They come after shadow justice secretary Robert Jenrick appeared to hint at a Tory-Reform pact at the next general election.
He was secretly recorded talking about the “nightmare scenario” of Labour winning the next election because the Tories and Reform are “disunited”.
He said: “I want the fight to be united. And so, one way or another, I’m determined to do that and bring this coalition together and make sure we unite as a nation as well.”
Tory leader Kemi Badenoch has insisted there will be no deals with her rival right-wing party, telling Sky News: “I’m not going into any coalition whatsoever with Nigel Farage or Reform at national level.”
But she was directly contradicted by Ben Houchen, the Tory mayor of Teesside, who later said: “If at the next election, there are a number of MPs in the Tory Party and Reform that created a significant majority then obviously there’s going to be a conversation to form a coalition or some sort of pact.”
A Labour Party spokesperson said: “Ben Houchen is saying what Kemi Badenoch only dares to hint at: the Tories and Reform will work together to stop Labour bringing the change this country needs.
