ATHENS, Greece — Investors holding Greek government bonds issued under foreign law face a Wednesday night deadline to decide whether to agree to swap them with new securities worth less than half their original value.

The government has already extended the deadline twice. The swap for private holders of bonds issued under Greek law was carried out last month. The exchange will complete the biggest debt writedown in history, relieving the crisis-hit country of just over half its $271.5 billion pile of debt held by banks, pension funds and other private investors.

The deal is intended to secure Greece’s long-term debt sustainability, bringing the amount owed to its creditors to around 120 percent of gross domestic product by 2020, from nearly 170 percent now.

Last month, Athens swapped bonds worth $232 billion issued under domestic law for new ones, forcing investors to take a cut of about 75 percent on the real value of their investment.

The debt reduction deal was essential for Greece to secure funds from a multibillion euro second international bailout, and to prevent the country from a potentially catastrophic default that could have dragged down the euro.