UPDATE 1-US Sen. Brown says won't back Wall St bill due to tax

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WASHINGTON, June 29 (Reuters) - U.S. Republican Senator Scott Brown, a key swing vote in the battle to overhaul financial regulations, said on Tuesday he would not back the final bill unless lawmakers drop a $19 billion tax on large financial firms.

"If the final version of this bill contains these higher taxes, I will not support it," Brown wrote in a letter to the Democratic lawmakers who have overseen the bill.

Brown's decision leaves Democrats two votes short of the 60 they need to clear a procedural hurdle and send the bill on to President Barack Obama to sign into law, unless they revise the bill to address his concerns.

Brown has previously supported the bill and helped Democrats pass it out of the Senate last month.

Negotiators from the Senate and the House of Representatives added the tax during an all-night negotiating session last week to cover the costs of the bill.

(Reporting by Andy Sullivan, editing by Vicki Allen)