Ed Miliband's net zero crackdown on landlords risks a £4,000-a-year rise in rents by the end of the decade, according to Tory analysis.
The Energy Secretary is planning to ban millions of owners from letting out their properties from 2028 unless they make costly green upgrades.
He is pressing ahead with the changes despite Rachel Reeves vowing to crack down on red tape heaping extra costs onto businesses.
Mr Miliband has even brought forward the planned deadline for the reforms by two years from 2030, meaning landlords have less time to prepare.
From 2028, all rental properties face having to achieve an Energy Performance Certificate (EPC) rating of C before new tenancies can be agreed.
Tory analysis of official government figures and estate agent estimates suggests the overall cost to landlords of the change would be £21.8 billion.
If owners were to pass all those costs onto their tenants then rents would rise by 5.1 per cent per year and 25.5 per cent by 2030. This would add up to £4,000 to the typical bill, the analysis shows.
Andrew Bowie, the shadow energy secretary, said: "Ed Miliband said he would cut energy bills by £300 but in just another broken promise bills are up and rents are set to follow, piling thousands in costs on hardworking families.
"The Conservatives are under new management and will tell the British people the truths Labour refuse to. You cannot keep piling costs on people without any consequences. He must change course now."
According to Savills, one of the UK's biggest estate agents, there are around 2.9 million privately rented homes which are not at a C rating.
Almost all of those are older properties, with more than seven in 10 houses built before 1950 below the new minimum standard.
Government figures show the average cost of upgrading a property to an EPC C rating is £7,529, though for the oldest homes it can be much higher.
That means private landlords face paying out £4.4 billion a year, equal to 5pc of the £85.6 billion they collect in rent, which could be passed on to tenants.
If they did so in full, the average UK rental contract would rise from £16,000 a year at present to £20,000 by the end of the decade.
A survey carried out by Shawbrook bank in 2022 found that more than half of landlords plan to pass on at least some of the cost to their tenants.
Property experts have separately warned that the green targets risk triggering a surge in evictions when they come into force.
They warned some landlords will simply "throw in the towel" and sell their houses, while others may kick out tenants while upgrades are carried out.
Mr Miliband has said the reforms champion tenants who have been "abandoned and forgotten as opportunities to deliver warm homes and lower energy bills have been disregarded and ignored".
The warnings come amid a struggle within Labour and even the Cabinet over the Chancellor's plans to prioritise growth above all else.
Ms Reeves has vowed to put future economic prosperity ahead of net zero targets and green initiatives, pitting herself against Mr Miliband.
A Department for Energy Security and Net Zero spokesman said: "Everyone deserves to live in a warm, comfortable home.
"That is why we are moving ahead with plans to require private landlords to meet higher energy performance standards.
"This could lift up to half a million households out of fuel poverty by 2030, while also making renters hundreds of pounds better off."