WASHINGTON, June 11 (Reuters) - The U.S. Securities and Exchange Commission on Thursday unanimously proposed to scrap longstanding Wall Street regulations requiring the execution of stock trades at the best available price, saying the rules drove up costs and complexity and were no longer necessary.

"I've opposed the trade-through rule since its inception and have elaborated on my concerns from this very stage," SEC Chair Paul Atkins said at a public meeting of the five-member bipartisan commission, which currently has only three Republican members.

(Reporting by Douglas Gillison in Washington)