British businesses now have to let trade union members into their offices to recruit after the Labour government’s overhaul of workers’ rights.

The new rules, starting Friday, are already causing employers to draft confidentiality agreements to allow union members access to workplaces, due to worries that union reps may get a glimpse of sensitive data on computer screens or make clients uneasy.

Others are updating their HR handbooks to prevent unrest among workers with different views about union membership. Companies that repeatedly block union access agreements face fines up to £500,000 ($666,000).

“I think it’s fair to predict there are going to be hundreds if not thousands of emails sent out at midnight,” said David Hopper, industrial relations partner at law firm Lewis Silkin. “There is going to be an immediate shock for a lot of the business community when they wake up this morning.”Trade unions are counting on these new access rights to recruit members and establish a presence in critical industries where they currently have minimal representation. Private sector union membership stands at just 12%, a significant drop from nearly 22% thirty years ago, and remains particularly rare among white-collar professionals such as software engineers, lawyers, bankers, and consultants.

Large businesses like Meta.com Inc. and John Lewis have already struck union access deals earlier this autumn, in anticipation of the rule changes. Unions are reaching out to everyone from financial services to logistics firms, with the average business receiving around three requests from separate unions for each of their sites, said Andrew Moore, employment and incentives partner at Addleshaw Goddard.

“This is really impactful,” said Moore. “This will change how businesses interact with their people.”

“The new access rights are a game changer,” said Mike Clancy, the general secretary of Prospect, the trade union that won the Meta access deal. He added that the new rules will help unions break ground into high-growth sectors like tech. “This is the biggest thing that would reverse the historic decline of the union movement in the private sector over the last few decades.”The changes come on top of reforms streamlining unions’ efforts to get a say in negotiations over pay, working hours and holiday, which came into force in April. Since then, applications for formal bargaining rights doubled year-on-year. Lawyers expect them to accelerate as union reps make their way into workplaces.

Letting unions in goes beyond office access. Some firms will ask union reps to undergo security clearances and safety training. Others are having to find special meeting rooms, away from client spaces, or cording off sections of common spaces like canteens, where unions can set up.

Because sharing employee contact details with unions violates data privacy regulations, employers are providing alternative solutions, such as granting unions advertising placement in companywide newsletters and permitting them to post QR codes on notice boards.

Companies have up to eight weeks to reach an access agreement with unions. Should both parties fail to reach an agreement, the Central Arbitration Committee —the body responsible for resolving workplace disputes — can intervene to establish the access terms.

The new rules set weekly access during working hours as a benchmark, either in person or digitally, although unions are likely to opt for less frequent visits. Representatives and management can set schedule dates in advance; otherwise, unions must issue prior notice before visiting the workplace.

“For businesses which haven’t interacted with unions before, this is going to be a big culture shift,” said Hopper. “It’s not just a culture shift for UK businesses and UK HR teams, but also potentially for their parent companies located around the world.”