The US government's review of a landmark 2010 financial reform law will not be complete by early June as originally targeted, and officials will now report findings piece-by-piece, with priority given to banking regulations, sources familiar with the matter said on Monday.

President Donald Trump has pledged to do a "big number" on the Dodd-Frank financial overhaul law, which raised banks' capital requirements, restricted their ability to make speculative bets with customers' money and created consumer protections in the wake of the financial crisis.

In February, Trump ordered Treasury Secretary Steven Mnuchin to review the law and report back within 120 days, saying his administration expected to be cutting large parts of it.

But the Treasury Department is still filling vacancies after the transition from the Obama administration and there are not enough officials to get the full review done by early June, three sources said.

A Treasury spokesperson dismissed the idea the report that would be broken up because the department is short-handed, saying the reach of the project could require several separate reports, as permitted under the executive order.

"Treasury has an entire team dedicated to reviewing the financial regulatory rules and will begin reporting our findings to the president in June," the department spokesperson said.

"Given the volume and scope of the issues we are reviewing that involve potential changes to the financial regulatory system, we are carefully considering the best options to begin rolling them out in the most effective and responsible manner," the spokesperson said.

The Treasury Department will first report back on what banking rules could be changed, including capital requirements, restrictions on leverage and speculative trading.

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Examinations of capital markets, clearing houses and derivatives as well as the insurance and asset management industries and financial innovation and banking technology will come later, the sources said.

It could be several months until these other stages of the financial reform review are completed, some of the sources said.

The piecemeal approach could create challenges for some sectors if parts of the report are significantly delayed. The report has been highly anticipated, as it marks the new administration's most detailed foray into outlining what it wants to do with financial rules.

Trump previously has spoken only in broad terms about easing regulation surrounding lending.

Any efforts to rework existing regulations or craft new legislation will be a lengthy and contentious process, something that banking lobbyists have said will make any delay to the administration's initial findings costly for businesses eager for regulatory relief.

Former BlackRock executive Craig Phillips is leading the administration's plan for financial deregulation. Alongside other Treasury officials, he is soliciting feedback from banking industry groups and executives for how banking policy should be shaped.

The change in the timing of the Treasury report comes after Trump ordered a separate review of some key planks of the Dodd-Frank financial reform law.

In April, Trump signed a pair of executive orders directing a review of two additional regulatory powers - orderly liquidation authority, which allows regulators to step in and wind down a failing financial institution, and systemic designation, in which certain large firms may be deemed critical to the overall health of the financial system, meriting stricter oversight.

The findings from those reviews are not expected until October.