Boris Johnson’s personal approval rating has slumped to its lowest level on record after his botched attempt to scrap Westminster’s standards system and spare a Tory MP from being suspended.

According to a new Opinium poll for the Observer, the prime minister’s personal ratings have now fallen to -20, down from -16 last week. It surpasses the previous low of -18 that Johnson recorded a month ago, suggesting he was already experiencing a low point in his popularity in recent weeks.

The Tory lead has also fallen to a single point in the past week, according to the poll that is the first to be conducted entirely after the resignation of Owen Paterson. The former Tory cabinet minister quit as an MP after No 10 ordered an embarrassing U-turn over its attempt to spare him from a 30-day Commons suspension for breaching lobbying rules.

There was also a significant shift in who voters see as the best prime minister. An 11-point lead for Johnson has shrunk to just 2 points. Johnson is regarded as the best prime minister candidate by 28% of voters, down 5 points, with the Labour leader, Keir Starmer, on 26% (up 4 points).

No 10 had backed an amendment suspending Paterson’s punishment and creating a new standards body with a built-in Tory majority. The next day, after intense criticism, the government backed down. It has caused fury among Tories at all levels of the party, many complaining of a backlash from constituents.

However, the Tories still lead in terms of overall voting intention at 37%, down 3 points on last week. Labour is up one point on 36%. The Lib Dems are on 9% and the Greens on 7%. Thirty per cent approve of the job Johnson is doing, while 50% disapprove. Starmer’s approval rating is unchanged at -9, with 29% approving of the job he is doing and 37% disapproving.

The results of the poll will anger Tory MPs and cause alarm in Downing Street. While the Tory conference was largely successful for the party, the recent budget caused some ideological splits. The row over Paterson saw whips having to persuade MPs in newly won former Labour seats to protect an MP in a safe, traditional constituency. It led to a major rebellion, with dozens of Tories abstaining rather than backing Paterson.

The Tories are regarded as more corrupt than other parties, with 39% saying a Conservative government would be more corrupt, and 14% saying Labour would be. However, 26% think they’d be as bad as each other.

As to Paterson’s punishment, 59% think the 30-day suspension was correct, 10% think it was wrong, and 31% did not know. A majority (60%) think the government was wrong to try to overturn the suspension.

“Johnson’s approval rating has been slowly slipping for months but this is the lowest we have ever recorded for him,” said Adam Drummond of Opinium. “By two to one, voters think the prime minister and the government and party he leads are corrupt rather than honest. The figures for Labour and Keir Starmer are much more positive and Johnson’s lead as ‘best PM’ has shrunk from 11 points to just two. Nevertheless, after another week of torrid headlines, the question is why Labour still haven’t managed to pull ahead.”

Opinium polled 1,840 people online between 5 and 6 November.