The telecoms giant has said that the government will look at the increased stake inBT by French billionaire Patrick Drahi.
Altice increased its investment from 12% to 18% in December, prompting speculation the firm could face a takeover bid.
The National Security and Investment Act gives the business secretary new powers to investigate.
The company said it would co-operate with the review.
The Business Secretary was exercising his call-in power under the National Security and Investment Act.
The new legislation gave the government the power to block acquisitions on national security grounds.
The government and the company are in the middle of a transformational programme to build a national broadband fibre network, a strategy crucial both to the company and the government.
In the morning trading in London, shares inBT fell more than 4%.
Mr Drahi began buying shares of the company in June of last year. His purchase of 12% of the company made him the largest shareholder.
The takeover rules would force him to make an offer for the whole company if he owned less than 30%.
There were rumors about other possible bidders.
The government said it would intervene if necessary after he increased his stake in the company.
Altice said that it had no plans to make a full bid.