In a rush to catch a flight at Singapore Changi Airport's Budget Terminal, 61-year-old retiree Ang Heng Soon, left his house with the wrong passport.

Instead, he took his 39-year-old son's passport, The Straits Times reported today.

Both passports were left on the dining table as his son was also flying out the same day on a business trip.

The honest mistake was only discovered later in the day, when Mr Ang had already cleared security checks at Changi Airport and flew to Vietnam.

Despite having the wrong passport, Mr Ang was able to check in at the Tiger Airways counter in order to board his flight to Ho Chi Minh City for his six-day holiday.

He, then, managed to clear past Certis Cisco officers at the entrance to the restricted area.

However, he did face some problems at the immigration checkpoint. The Automated Clearance System failed to pick up his fingerprint scan and so the Checkpoints Authority told him to go through the manual clearance instead. Despite this diversion, an officer cleared him to leave Singapore and he boarded his flight.

Mr Ang told The Straits Times he only realised his mistake during the flight.

Once he arrived in Ho Chi Minh City, he informed immigration authorities there and they sent him back immediately on the same plane back to Singapore.

Around the same time in Changi Airport's Terminal 1, Mr Ang's son, Vincent, discovered he had his father's passport and cancelled his flight to Hong Kong.

Speaking to The Straits Times, Mr Vincent Ang, said, "The question is, how did this happen? From a security point of view, this is pretty shocking."

Responding to queries from The Straits Times, the Immigration and Checkpoints Authority confirmed that Mr Ang had cleared all the checks at the airport despite showing his son's passport.

In a statement, it said that the immigration officer who looked at his passport and did the 'face-to-face verification' let him through because he bore a resemblance to the photo in the passport.

The spokesman apologised to Mr Ang for the inconvenience caused and said: "ICA takes a serious view of such lapses.

"We will conduct a thorough investigation into the case. Appropriate disciplinary action will be taken against the officers responsible for the lapses."

As for the security officer, ICA said that he should not have relied only on resemblance to the photo in the passport but, "should have checked Mr Ang's boarding pass with his passport."