President Barack Obama hit out at corporate “inversions” - the practice of US companies moving overseas in tax-avoiding paper-based mergers - calling them “one of the most insidious loopholes out there.”

Speaking a day after a third round of restrictions aimed at curbing the practice were unveiled, Mr Obama accused the companies of enjoying the rewards of being American “without fulfilling the responsibilities to pay their taxes the way everybody else is supposed to pay them.”

“When companies exploit loopholes like this, it makes it harder to invest in the things that are going to keep America’s economy growing strong for future generations,” the president said at a specially convened press conference to highlight the actions.

“It sticks the rest of us with the tab and it makes hard-working Americans feel like the deck is stacked against them.”

For the first time Mr Obama also addressed the leak from a Panama law firm showing the concealment of assets by wealthy individuals, saying that it was another reminder that “tax avoidance is a big global problem.”

Mr Obama called again for US lawmakers to pass legislation to end the inversion loophole fully, saying “only Congress can close it for good.”

Shares in Dublin-based pharmaceutical company Allergan plummeted after analysts said the new rules could kill off the $160 billion tax-driven inversion of US drugs giant Pfizer into the Irish company.

“It looks like very much a targeted strike,” said one Irish corporate source who has advised US companies on past inversion transactions.

The deal was designed to help Pfizer reduce its taxes by moving the company’s domicile to Ireland to avail of the Irish corporate tax rate of 12.5 per cent, instead of the top US corporate rate of 35 per cent.

The measures unveiled by the US Treasury and Internal Revenue Service on Monday target in particular what the US administration called “serial inverters” - foreign companies that have grown in size in the past three years through multiple inversions.

Allergan is the amalgam of four merged firms created via inversions over the past three years: the 2013 purchase of Irish drug maker Warner Chilcott (formerly Galen) by New Jersey-based Actavis, the 2014 acquisition of US firm Forest Laboratories by Dublin-based Actavis and the $66 billion takeover of California-based Allergen in 2015 by Actavis.

The Treasury’s new rule imposing a three-year limit on foreign companies bulking up on US assets to avoid ownership rules for future inversions would exclude these three deals from the Pfizer transaction.

Another restriction is aimed at ending a tactic known as earnings stripping. This involves the newly created overseas parent company stuffing US subsidiaries with debt in the inversion process allowing them to use interest payments to reduce taxable American profits and to keep the one-time US firm’s global earnings away from the American taxman.

The Irish source described this as “a nice kicker to all of these deals.”

Republican presidential frontrunner Donald Trump called inversions “a big problem” but criticised Mr Obama’s latest measures to stop them.

“We have so many companies leaving us. It is disgraceful and I’ll stop it,” he said during a campaign stop in Wisconsin.

“The politicians don’t know how to stop it. They don’t have a clue.”