A weekly presidential preference poll that showed higher support ratings for former Prosecutor General Yoon Seok-youl compared with other surveys was stopped this week, prompting Yoon to blame political pressure from other candidates and their supporters.

PNR Research, the pollster that had conducted the survey every week since April, said its clients told it last week that they would no longer commission it for the poll.

Yoon said in a statement on Tuesday that the PNR poll was stopped because ruling Democratic Party supporters protested, and he called on the national election watchdog to look into what happened.

Money Today, the online outlet that had commissioned the PNR poll, said there had been no external pressure or intervention to stop it.

“Some media reports and suspicions raised by the Yoon Seok-youl camp did not go through any confirmation from our side,” it said in a statement.

Money Today expressed “regrets against speculation and attacks based on such speculation,” and demanded that “such behavior of dragging the media into a political fight does not recur.”

The PNR poll always showed Yoon ahead of his rivals, including Gyeonggi Gov. Lee Jae-myung, while Lee beat Yoon in some other recent surveys.

A weekly poll from PNR just after Yoon’s mother-in-law was sentenced to prison July 2 showed that support for Yoon had actually climbed from the previous week, though his ratings tumbled in all other surveys.

About the varying poll results, PNR said they depend on how the polls are conducted -- that is, whether the pollsters use automated response systems or telephone interviews.

PNR chose automated response systems exclusively for all of its polls.

A PNR official said automated response systems produced more accurate results as only people interested in politics were likely to respond to prerecorded questions.

The results of a poll released Monday showed for the first time that even former Democratic Party leader Lee Nak-yon was ahead of Yoon in a hypothetical two-way race, within the margin of error.

A survey of 1,011 voters conducted over the weekend by Wingkorea, commissioned by Asia Business Daily, showed that if they had to choose between Yoon and Lee Nak-yon, 43.7 percent said they would vote for Lee and 41.2 percent picked Yoon.

In another survey, released Wednesday, Lee Jae-myung beat Yoon in an imaginary two-way race, outside the margin of error.

In the poll of 1,001 voters conducted from Saturday through Monday by Hangil Research, commissioned by Kuki News, 43.9 percent chose Gov. Lee and 36 percent picked Yoon in a hypothetical two-way race.