RBS may seal branches, WorldPay sales Tues -source

* Santander set to buy 318 branches for up to 1.8 bln stg

* Advent/Bain nears deal for WorldPay for 2-2.5 bln stg

LONDON, July 30 (Reuters) - Royal Bank of Scotland (RBS.L) could complete the sale of a network of UK branches and its payment processing business as early as Tuesday, fetching up to 4.3 billion pounds ($6.7 billion), a person familiar with the matter said.

Santander (SAN.MC) is near to buying 318 branches being sold by part-nationalised RBS for up to 1.8 billion pounds, sources have said, and one said a deal could be sealed by Tuesday.

The Spanish bank said on Thursday it was in the last stages of talks on the deal. [ID:nLDE66R1ML]

RBS, 83 percent owned by the UK taxpayer, could announce the sale of its payment processing business to private equity firms Advent International and Bain Capital for between 2 billion and 2.5 billion pounds on the same day, three sources said.

RBS has been told to sell both businesses as a cost of taking billions of pounds in rescue funds from the UK government.

Stephen Hester, RBS chief executive, is shrinking the bank's balance sheet to focus on its core strengths after an aggressive expansion by his predecessor over the previous decade.

RBS has been in exclusive talks with Santander and Advent/Bain on the respective deals, and has been keen to seal the deals before Friday's results. [ID:nLDE66M1HV]

The bank is expected to report a second-quarter loss, after returning to operating profit in the first quarter, as investment banking income slowed across the industry in the second quarter from the previous three months. ($1=.6402 Pound) (Reporting by Steve Slater and Victoria Howley; Editing by Jon Loades-Carter)