A software glitch that crept into a massive system integration project at Japan's Bank of Tokyo Mitsubishi UFJ left thousands of customers unable to withdraw money on Monday morning.

Customers of the bank were unable to complete about 20,000 transactions when they used ATMs belonging to Seven Bank, an electronic bank operated by convenience store chain Seven Eleven, said Takashi Takeuchi, a spokesman for the Bank of Tokyo Mitsubishi UFJ. The problems were caused when the Bank of Tokyo Mitsubishi UFJ system sent a message containing a Chinese character that the Seven Bank system was not expecting.

When Bank of Tokyo Mitsubishi UFJ customers attempt to withdraw money, one of the checks that is made it whether their bank account books -- notepad size books used by all major banks in Japan to record transactions in lieu of monthly statements -- are up to date. If there are 10 or more transactions waiting to be recorded in the book then the ATM will remind customers to update their bank account books.

But a single Chinese character in the message asking customers to update their books sent from the Bank of Tokyo Mitsubishi UFJ's computer was not understood by the Seven Bank ATM. The ATM was expecting a Japanese katakana character, not a Chinese character, and that caused the transaction to fail, said Takeuchi. Withdraws at other ATMs and at Seven Bank by those customers whose books didn't need updating were carried out with no problem, he said.

Problems began soon after 7 a.m. local time, when the Bank of Tokyo Mitsubishi UFJ ATM network was turned on again after an upgrade to the bank's host computer. Service was returned to normal at 11:55 a.m. and Bank of Tokyo Mitsubishi UFJ issued an apology to its customers.

Bank of Tokyo Mitsubishi UFJ was created in January 2006 through the merger of Bank of Tokyo Mitsubishi and UFJ Bank. It ranks as the world's largest bank by assets, and integration of its two computer systems for retail banking has been put off until now because of the monumental task facing integrator IBM Japan.

The weekend work saw the Bank of Tokyo Mitsubishi's system, which was originally supplied by IBM Japan, upgraded. The next stage of the year-long project will see the UFJ Bank system, originally supplied by Hitachi, upgraded in a series of five steps between July and December. When the upgrades are complete, the two systems will be merged to create a single system.