First Complete, formerly Home of Choice before it was rescued from administration by LSL last year, plans to retain the TMA brand.
It said it will expand TMA’s current proposition to directly authorised (DA) mortgage and insurance brokers over the coming months.
TMA launched in 1999 and supports more than 3,500 DA mortgage advisers.
The deal is the latest of several acquisitions by LSL since the beginning of 2010, as it aims to match ambitious growth plans.
Jon Round, chief executive of First Complete, said: “The Mortgage Alliance is a mortgage club with a good reputation, which has an exceptional offering to directly authorised advisers.
“First Complete has gone from strength to strength in the last year, providing an excellent service to its appointed representatives; now First Complete will also be able to provide a high quality proposition to directly authorised firms.”
A spokesperson for Santander said: “Santander is committed to becoming the best commercial bank in the UK. Following a strategic review and in line with our focus on core business areas, we can confirm that we have taken the decision to sell The Mortgage Alliance to the LSL Group.”