Keir Starmer has issued a warning over the "dangerous right-wing politics" of Reform UK.
The PM argued the party is "not the answer for Britain" as a poll showed last week Nigel Farage's party taking a lead over Labour and leapfrogging the Tories.
In a major speech to the Scottish Labour conference in Glasgow, Mr Starmer said: "If we don't show a path to the future, others will fill that void.
"In fact they already are. A dangerous right-wing politics - even here in Scotland - that says they are the ones that can tilt politics towards the interests of working people".
But Mr Starmer promised to be ready for the "test" and highlight Reform UK's "bluster". He cited the decision of all five of Reform UK's MPs to vote against the landmark Employment Rights Bill in the Commons.
The legislation spearheaded by Deputy PM Angela Rayner promises to boost workers' rights with a range of measures and boosting sick pay provision. The PM said: "You want to know what Farage and Reform are doing, on their rare visits to Parliament?
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He added: "They're voting against our employment rights Bill. They talk the language of workers' rights online and on the doorstep, but they voted against banning fire and rehire.
"And what about the NHS? They want to charge people to use our NHS. They claim to be the party of patriotism but they are fawning over Putin. No, they are not the answer for working people in Britain. And it will never be the answer for Scotland."
Mr Starmer also announced at the Scottish Labour conference the UK government would provide an extra £200 million of funding for Grangemouth as the oil refinery in the area is set to close.
The PM pledged the cash from the National Wealth Fund, which he said would be an "investment in Scotland's industrial future".
He said: "We will grasp the opportunities at Grangemouth, work alongside partners to develop viable proposals, team up with business to get new industries off the ground and to attract private investors into the partnership we need.
"An investment in Scotland's industrial future, that's the difference a Labour Government can make because we all know what happens if we don't win this race, it's written into the fabric of every community in this nation, the skilled jobs that disappeared, the communities that lost their place in the world. Not just the security and dignity of their work, not just their pride and purpose, their identity."