UPDATE 1-UK bank B&B to detail rights allocation - source

(Adds detail, background)

LONDON, July 4 (Reuters) - British bank Bradford & Bingley BB.L will announce later on Friday the allocation breakdown for the rights issue previously underwritten by U.S. private equity firm TPG Capital, a source familiar with the matter said.

TPG [TPG.UL] had planned to buy a 23 percent stake in B&B for 179 million pounds ($355.4 million), but pulled out late on Thursday after ratings agency Moody's cut the buy-to-let mortgage lender's debt rating.

B&B will increase to size of its rights issue to 400 million pounds ($794 million) from 258 million pounds in a deal underwritten by Citigroup (C.N) and UBS (UBSN.VX).

Key shareholders, including Standard Life (SL.L), M&G (PRU.L), Legal & General (LGEN.L) and Insight, which hold a combined stake of approximately 14 percent, were supporting the enlarged rights issue, B&B said. (Reporting by Daisy Ku; Editing by Erica Billingham)