By Mark Felsenthal

WASHINGTON (Reuters) - U.S. bank regulators breathed a huge sigh of relief in early April when improving financial markets looked set to push the nation's 19 largest banks through the gauntlet of tough "stress tests" in reasonably good shape.

In early March, a month after Treasury Secretary Timothy Geithner announced the tests on February 10 to help restore investor confidence in the major banks, the scene was much bleaker: major stock indexes had slumped to 12-year lows on persistent fears about the financial weakness. It looked possible that a major bank might need an emergency government rescue even before the stress test results could be announced.

But since the trough, markets improved steadily with rising share prices and volumes, better liquidity and other signs of stabilization, and regulators gained comfort that capital markets would be willing to fill any holes the stress tests unearthed at banks.

"It looked to us by mid-March, and early April, that this might work," said a senior U.S. regulatory official, who spoke on condition of anonymity because of the sensitivity of the tests.

Stress tests released on Thursday showed the largest banks had a $74.6 billion (49.4 billion pound) gap to make up to ensure they could withstand a worst-case scenario regulators set for them. Nine of the 19 banks tested already had the necessary buffer and would not need to raise any more.

Morgan Stanley and Wells Fargo sold more than $15 billion of shares and bonds the next day, and Bank of America Corp announced plans to sell 1.25 billion shares as investors reacted positively to the relatively small size of the shortfall.

If by the middle of next week banks are half-way to raising the needed capital, and a half dozen or so of the banks needing to augment their buffers have done so, regulators would be pleased, the official said. In particular, such a development would likely signal the administration would not have to ask Congress to replenish its bailout war chest.

TEST IDEA TOOK SHAPE IN SEPTEMBER