By Matt Scuffham

(Reuters) - TD Ameritrade <AMTD.O> has agreed to buy Scottrade for $2.7 billion in a deal that would bring together two of the biggest U.S. discount brokerages, but is expected to face scrutiny from regulators.

It is the latest in a wave of consolidation in an industry which has been grappling with intense competition and weak trading volumes as a result of small investors being drawn towards cheaper investment products that track major indexes.

E*Trade Financial Corp <ETFC.O>, another discount broker, said in July that it would buy online brokerage OptionsHouse for $725 million, while Ally Financial Inc <ALLY.N> purchased TradeKing Group for about $275 million a month earlier.

In a connected deal, Toronto-Dominion Bank <TD.TO>, TD Ameritrade's biggest shareholder, said on Monday it had agreed to buy Scottrade's banking business for $1.3 billion as it continues to ramp up its expansion in the United States.

TD Ameritrade's acquisition of Scottrade combines two of the United States' "big five brokerages", the others being Charles Schwab, Fidelity Investments and E-Trade, and would leave only four major brokers operating in the marketplace.

"I think that the authorities in the United States are unlikely to let this pass without a pretty close look," said John Briggs, an antitrust attorney with the law firm Axinn, Veltrop & Harkrider. "I think the transaction deserves scrutiny and will get scrutiny."

TD Ameritrade's CEO Tim Hockey said he believed regulators would look at the deal "fairly."

"I still think this is a considerably competitive marketplace, that’s for sure. There are lots of opportunities for additional competitors to get into our space and continue to drive price competition," he said.

The deal will produce a combined business with around 10 million client accounts and $1 trillion in assets, which will execute around 600,000 trades per day.

The purchase prices comprises $1.7 billion in cash and $1 billion in new shares. It will net a windfall for Scottrade's co-founder and Chief Executive Rodger Riney, who set the company up in 1980. Riney, who said last year he was being treated for cancer, will join the TD Ameritrade board and also become TD Ameritrade's fourth biggest shareholder.

"The environment has become more challenging than ever and we’ve faced some of our own internal pressures. As the years have gone by, it has become apparent that scale is more important than ever," Riney said on Monday.

In addition to investors switching from buying individual stocks to holding cheaper index-tracking funds, discount brokerages have also been facing price competition from full-service brokerages and automated investment advisers, known as robo advisers. However, new regulations on giving retirement advice in the United States are expected to benefit discount brokerages because investors may switch to them from the full-service brokerages to avoid fees.

Hockey said TD Ameritrade will look at further deals which "make strategic and financial sense" in addition to Scottrade.

TD Ameritrade, 42-percent owned by Toronto Dominion Bank, said it expected annual cost savings of $450 million as a result of the deal, with another $300 million of potential savings identified in the longer term.

As part of those plans, Hockey said around 25 percent of the combined business's 600 branches will be closed. TD Ameritrade currently has 100 branches while Scottrade has 500. Hockey said in the interview the combined workforce of 10,000 will be reduced by about 20 percent.

TD Bank has been expanding aggressively in the United States, growing both its retail and investment banking divisions as it looks to diversify from its home market. It has already grown to be one of the 10 biggest banks in the United States and has a major retail presence with 1,300 branches.

Shares in TD were up 0.3 percent with TD Ameritrade down 3.3 percent in mid-day trade.

(Additional reporting by Sruthi Shankar, Richa Naidu and Sangameswaran S in Bengaluru; Editing by Lisa Von Ahn and Nick Zieminski)