The U.S. will roll out a new regulation on Wednesday that would deny asylum to most migrants crossing the U.S.-Mexico border illegally, a key part of President Joe Biden's enforcement plan as COVID-19 border restrictions known as Title 42 end this week.

The final version of the regulation, to be posted online later in the day, will have no major changes from a draft published in February, a senior Biden administration official told reporters on Tuesday evening.

The regulation will create a new presumption that migrants arriving at the border are ineligible for asylum if they passed through other nations without seeking protection elsewhere first or if they failed to use legal pathways for U.S. entry.

The new restrictions will apply to the vast majority of non-Mexican migrants since they typically pass through multiple countries en route to the United States.

The Biden administration is preparing for possible increase in already record levels of unauthorized border crossings when the COVID-19 restrictions, first implemented in March 2020, are lifted on Thursday. Migrants have been amassing in Mexico this week as thousands crossing into the U.S. have strained border cities.

The Title 42 restrictions allow U.S. authorities to rapidly expel many non-Mexican migrants to Mexico without the chance to seek U.S. asylum. Mexicans, the nationality most frequently caught crossing, are able to be quickly returned to Mexico under bilateral agreements that predated the COVID-19 restrictions.

Republicans have criticized Biden, a Democrat running for re-election in 2024, for rolling back the hardline policies of Republican former President Donald Trump, the current frontrunner for his party's nomination.

Biden's new regulation restricting asylum access at the border resembles similar measures implemented under Trump that were blocked by U.S. courts. The move also counters previous statements Biden made in 2020 on the campaign trail, saying he thought it was "wrong" for people not to be able to seek asylum on American soil.

Some Democrats and immigration advocates have said the regulation undercuts the ability to seek asylum at U.S. borders as required by U.S. law and international agreements. The American Civil Liberties Union has already signaled it will sue over the Biden policy.

On the other side of the ideological spectrum, a coalition of 22 Republican state attorneys general separately opposed the measure, saying that it is "riddled with exceptions."

In addition to the bar on asylum seekers, which could ramp up deportations, Biden officials said in late April that they are expanding legal pathways for migrants abroad in order to provide alternative ways to enter the United States and discourage illegal crossings.

On the call with reporters on Tuesday, Biden officials said the administration planned to open more than 100 migration processing centers in the Western Hemisphere and would launch a new online appointment platform in the coming days.

The officials also said they expected Mexico to step up immigration enforcement this week, including in southern Mexico.

(Reporting by Ted Hesson in Washington and Kristina Cooke in San Francisco; Editing by Mica Rosenberg, Aurora Ellis and Jamie Freed)