Reform UK is a growing threat for Sunak. Since Lee Anderson's decision to defect to the party, its credibility has only inflated - and Sunak's has fallen even further.
While it tries to avoid being branded as a one-issue party, it is undeniable that a large part of its appeal stems from concern about migration, both legal and illegal.
The perception that Sunak has failed to get a grip on boat crossings and net migration figures is turning people away from the Tories and towards Richard Tice's party, which pledges to "reclaim Britain".
While the Prime Minister has repeatedly used strong language to describe his plan for migration, with his slogan 'Stop the Boats' leaving him absolutely no wiggle room, he has failed to match it up with his actions.
Reform UK is a growing threat for Sunak. Since Lee Anderson's decision to defect to the party, its credibility has only inflated - and Sunak's has fallen even further
PA
His Rwanda Bill is still being ping-ponged back and forth between the Lords and the Commons, while record numbers of migrants arrive on British shores.
A total of 5,435 migrants have arrived this year so far, significantly more than the 3,793 who had crossed the Channel by small boat at this point last year.
And Sunak's failure to get a grip on the issue in a credible way is playing straight into Reform UK's hands.
But there is apparently a solution, new research has suggested.
A survey conducted by the Legatum Institute found that Reform's supporters are largely made up of disgruntled Tories who believe the UK is worse than it used to be. Crucially, it found that some 42 per cent would consider backing Sunak if he took hardline action on migration - including holding a referendum on cutting migration levels to 100,000 a year.
The report warned that the issue of migration is "paramount" to the party's success, saying that if the concerns are not addressed, it could continue to take votes from the Tories for years, or even decades, to come.
While holding a referendum on the issue could be perceived as yet more lip service to tackling migration, such a solid commitment to cutting numbers could demonstrate a commitment to the issue that is currently lacking.
But realistically, while Sunak could use a referendum to prove his commitment to the problem, it still wouldn't answer the long-term question of how to successfully cut numbers.
If he can't find a working, ethical solution to something that so many people are now concerned about, the problem will still be there.
Promising a referendum could help them in the polls in the lead-up to an election, but realistically in the long term, it would need to be backed up by action to avoid making the problem worse.