LONDON — Investors are increasingly sounding the alarms over Monte dei Paschi di Siena, the beleaguered Italian lender, as concerns mounted on Wednesday about its ability to raise new capital and avoid a government bailout.

The troubled Italian banking system, stuffed with dubious loans, has broadly put global policy makers and investors on alert for signs of a potential shock. And Monte dei Paschi, the country’s oldest bank, is in the weakest spot.

The bank, which was the worst performer in the European stress tests this year, was given until the end of the month to raise 5 billion euros, or about $5.2 billion. But investors do not seem convinced that the bank can find the funds, sending shares plunging on Wednesday.

Thursday is the deadline for investors to agree to purchase new shares in the lender as part of its recapitalization plan, but news reports have indicated that Monte dei Paschi has struggled in its efforts to sell shares.