Brighton Housing Trust has bought into the public relations business in an attempt to boost the charity's income.
Andy Winter, chief executive of BHT, declined to disclose how much the charity had spent on buying the agencies, but said it was less than £95,000, which is 1 per cent of its annual turnover.
He said the charity would claim Gift Aid on the PR firms' profits, which would be directed to the housing trust.
The housing trust has set up a trading arm, BHT Enterprises, which owns the firms. Winter said he hoped the body would own other social enterprises in future.
"When the firms' owner first suggested that we buy them, I was sceptical, and so was our finance director," Winter said. "But we have realised that it makes good commercial sense."