According to the research group Consumer Intelligence Research Partners (CIRP), iPhone 8 made up 23 per cent of iPhone sales in the first quarter of 2018 and iPhone 8 Plus registered 21 per cent.
"The iPhone X trailed at 16 percent, down from 20 percent the previous quarter," CNET reported on 24 April, citing the survey's findings.
The survey also showed a trend among respondents toward buying past years' iPhones.
"During the same months of 2017, the brand new iPhone 7 and iPhone 7 Plus accounted for nearly 70 percent of iPhone sales. This year, a lot more people are buying iPhones from one, two and even three years ago," the report noted.
Apple will announce its next quarterly earnings report on 1 May.
In March, a top analyst with asset management firm Piper Jaffray said that unlike what Apple had expected, the response to iPhone X has been lukewarm as many existing users believe that the device is too expensive.
"According to Apple news site PED3.0, Michael Olson, a senior researcher with Piper Jaffray found that 44 percent people who didn't upgrade said that they baulked at the idea because their current 'iPhone works fine'," Fortune reported.
More than a third of the respondents said that the iPhone X, which starts at $999, is too expensive.
"And eight percent said they would have upgraded if the new phone had a larger screen," the report added.
Ming-Chi Kuo, an analyst with KGI Securities, said in March that the Cupertino-headquartered company would cease production of iPhone X mid-year.
Apple is reportedly gearing up to introduce three new iPhones in 2018 and one of them will be more popular than the others, according to Kuo.