Ministers have been warned they must ramp up the “scale and pace” of replacing fossil fuel boilers with heat pumps or Scotland’s net zero ambition will not be met.

Audit Scotland has reviewed the Scottish Government’s heat in buildings strategy, being led by Green Zero Carbon Buildings Minister, Patrick Harvie.

Mr Harvie is aiming to end fossil fuel heating systems in buildings by 2045 with certain trigger points for homeowners and businesses to replace traditional gas boilers with heat pumps or other renewable systems.

But concerns have been raised about the pace of progress with Mr Harvie forced to cancel an ambition to decarbonise 1 million homes by 2030.

The Audit Scotland report has warned that “the scale of the challenge of reducing emissions from heating homes is huge”, adding that “there are several risks to success”.

It adds: “Unless the scale and pace of activity significantly increase, the Scottish Government’s ambition will not be met.”

It is estimated that the Heat in Buildings Strategy, introduced in 2021, could cost £33 billion to the public sector, businesses and households but the Scottish Government has only committed £1.8 billion of public money in this parliamentary term.

The report urges ministers to ensure any investment has “maximum impact” and is “value for money” given the tough financial situation facing the public.

A significant issue highlighted by auditors is the almost two years spent by officials to build a team to deliver it.

Auditor general for Scotland, Stephen Boyle, said ministers should have addressed the capacity needs sooner. But since early 2023, “good progress” has been made in preparing to implement regulations.

Ministers have also been warned that success will depend on measures including providing financial support to homeowners and securing public finance and supply chain capacity.

The report noted the work for this is currently at an “early stage”, with officials told to increase the pace before legislation can be put to the Scottish Parliament.

Mr Boyle said: “Getting most households in Scotland to change to low carbon heating systems is a huge challenge. It is complex and relies upon a range of stakeholders and partners, including the public, the private sector and the UK Government.

“The Scottish Government now needs to carefully consider how to maximise its public spending and set out a clear delivery plan. It also needs to help the private sector to roll out funding deals that will support people to change how they heat their homes.”

Mr Harvie stressed the report “acknowledged that good progress has been made so far”, but also “rightly highlighted the huge scale of the challenge to move to clean heat in our homes”.

He added that his “ambitious policy proposals” represent “a step change in the way we heat our homes and buildings”.

Mr Harvie added: “I urge everyone to respond to our consultations to help us shape the heat in buildings bill that we are committed to bringing forward during this parliament.

“At the same time, we offer a very broad range of delivery programmes to provide advice and support for property owners related to energy efficiency and clean heating.

“We have committed to allocate at least £1.8 billion over this parliament to help kick-start growth in the market and support those least able to pay.”

But campaigners want Mr Harvie to accelerate his legislative plans.

Fabrice Leveque, energy policy manager at WWF Scotland, said: “The Scottish Government has rightly prioritised the heat transition, but this report is a wakeup call, showing that there is no time to waste if we're serious about delivering lower carbon, lower cost heating for households and businesses.

“That’s why we need to see a heat in buildings bill this summer, to give businesses the confidence to invest, and tradespeople develop the right skills to deliver the transition away from fossil fuel heating.

“The sooner we begin this transition, the sooner we can see homeowners enjoy lower energy bills and warmer homes, and Scotland can play its role in tackling climate change by ending a key source of our emissions.”

Scottish Conservative shadow net zero, energy and transport secretary, Douglas Lumsden, claimed the report was “a damning verdict on Patrick Harvie’s shambolic plans”, adding the strategy “will put enormous costs and conditions on Scottish households”.

“Audit Scotland point out that the SNP-Green government has no chance of meeting its own targets unless there is a substantial increase in pace and activity, and warns of ‘significant risks’ ahead.

“This is despite years of preparation and an estimated final bill of £33 billion. It shows how detached from reality Patrick Harvie’s plans always were.”

Scottish Labour net zero spokesperson Sarah Boyack, said the report “lays bare the chaos engulfing the SNP’s plans”.

“With energy bills soaring and fuel poverty rife, it is more urgent than ever that the SNP sets out a clear plan to upgrade homes and modernise heating systems.”