One of the biggest unions representing BT workers has urged the business secretary to ensure any potential takeover bid protects the telecoms company’s 100,000 staff, its hundreds of millions in research spending and the national interest.

Prospect wrote to Kwasi Kwarteng days before BT’s largest shareholder is allowed to make a potential offer under UK takeover rules.

Patrick Drahi, the French-Israeli telecoms billionaire known for cost-cutting at businesses he controls, is free to make his next move from 11 December with the expiry of a no-bid clause that was triggered when he took his £2.2bn stake in June.

Mike Clancy, the general secretary of Prospect, said: “The business secretary must take personal charge of making sure that any proposed takeover supports growing that investment, not a flight of money or skills overseas. We need a clear focus on protecting both our national interests and jobs while boosting private sector research and development (R&D), all of which could be at risk with a controversial takeover.”

Prospect said that any acquirer of BT, which is one of the UK’s biggest private sector tech and R&D investors, spending £720m annually, could seek quick profits through cuts to jobs and research focusing on “a short-term pay out rather than long-term returns”.

Clancy said: “Media reports over BT’s future ownership will unsettle staff in the run-up to Christmas, especially given the amount of change that the company has seen and continues to experience. BT is one of the jewels in the crown of UK innovation and tech R&D, despite a tough economic environment.”

There are growing concerns about the threat to Britain’s economy and national security from a string of recent takeovers of UK companies by foreign rivals and private equity firms.

Any move on BT is certain to result in scrutiny from the government, which has already ordered a number of investigations into nationally sensitive deals including the $54bn (£40.6bn) takeover of the Cambridge-based chip designer Arm by its US rival Nvidia.

From January, the government will gain tougher powers to block the takeover of key national assets under the National Security and Investment Act 2021.

Drahi may consider options less controversial than mounting a full-scale takeover bid. For example, he could attempt to increase his direct influence by asking for a seat on BT’s board, like Deutsche Telekom, BT’s second-largest shareholder.

He could also move towards increasing his stake by looking to acquire the 12% controlled by DT, which it has held in a passive capacity since as a legacy of BT’s takeover of the mobile operator EE in 2015.

Tim Höttges, the chief executive of DT, has said he “entertaining all options” regarding BT’s future.