LONDON — Tata Steel of India agreed on Monday to sell part of its British business to the investment firm Greybull Capital, preventing thousands of job losses and freeing it up to focus on the sale of its other big plant in Britain.

The British government has been under pressure to ensure that the Tata plants are sold rather than shut down after the steel maker, one of the world’s biggest, said on March 30 that it would sell its unprofitable British business, possibly putting 15,000 jobs at risk.

Greybull said it would buy Tata’s Long Products Europe division in Scunthorpe, in Northern England, which employs 4,400 people. Separately, the process to try to find a buyer for Tata’s other major British plant, at Port Talbot in Wales, began on Monday.

Greybull said it was arranging a 400 million pound, or $565 million, investment and financing package for the Scunthorpe business, as part of a deal that included an agreement with suppliers and trade unions.