Southern Cross has acquired the Ashbourne Group for an undisclosed sum, making it the largest care company in the UK.

The Darlington-based company said it would continue to make acquisitions in a bid to consolidate the sector.

Its national workforce has grown to 30,000 - and during the past two years, its head office staff numbers in Darlington have tripled, from 40 to 120.

Southern Cross announced in August it was committing its future to the town with the building of a four-storey head office in Archer Street, which will be completed in February.

For the moment, the company is based in the town's Valley Street. Support and administration staff for the company's homes will be based in the new offices.

The company has undergone a period of rapid growth during the past few years. It specialises in care for the elderly and for people suffering from dementia, as well as those with learning difficulties and brain injuries.

The company has 573 care homes across the UK, making it the largest provider of long-term residential and nursing care services. A year ago, it operated only 160 care homes.

Southern is owned by the Blackstone Group, a US private investment company. Blackstone bought Southern from West Private Equity last September for £162m.

Southern Cross director and Blackstone partner Joe Baratta said: "When we acquired the Southern Cross group in 2004, our strategy centred on leading the much-needed sector consolidation.

"Over the past 18 months, we have built the UK's largest and most diversified operator of care facilities."

Philip Scott, chief executive of Southern Cross, said: "The Ashbourne Group is a quality portfolio that, when added to our existing business, will further improve our reputation.

"While the sector still offers some real operational challenges, trading conditions are now such that we have been encouraged to accelerate both our acquisition and development activity."

He said there were further, smaller acquisitions planned.