Under the transition plan developers will get the two-and-half-year transition period starting from when the details of the new rules are published in Approved Document B.

For already approved applications, developers will be given 18 months to start pouring concrete to prevent stockpiling of pre-approved schemes.

If they fail plans will have to be redrafted to conform with the new rule and resubmitted.

The arrangements are hoped to avoid the shock wave sent through the London market when the Mayor Sadiq Khan pressed forward more rapidly causing schemes to be delayed by up to 9 months or even monthballed.

In a written statement, Michael Gove, secretary for housing and levelling up, said:  “I can now announce the intended transitional arrangements that will accompany this change to Approved Document B.

“From the date when we publish and confirm those changes to Approved Document B formally, developers will have 30 months during which new building regulations applications can confirm to either the guidance as it exists today, or to the updated guidance requiring second staircases.

“When those 30 months have elapsed, all applications will need to conform to the new guidance.

“Any approved applications that do not follow the new guidance will have 18 months for construction to get underway in earnest. If it does not, they will have to submit a new building regulations application, following the new guidance.”

He added: “This means that, for some years yet, we will continue to see 18m+ buildings with single staircases coming to the market.

“I want to be absolutely clear that existing and upcoming single-staircase buildings are not inherently unsafe.

“They will not later need to have a second staircase added, when built in accordance with relevant standards, well-maintained and properly managed. I expect lenders, managing agents, insurers, and others to behave accordingly, and not to impose onerous additional requirements, hurdles or criteria on single-staircase buildings in lending, pricing, management or any other respect.”