Mr Lew said the tax change should take effect retroactively from May 2014 – the time when Pfizer, the US drugs company, was considering a relocation to the UK in conjunction with its $106bn bid for AstraZeneca, a British rival, that ultimately collapsed.  More from Financial Times:Defensive play in the World Cup of corporate taxation US bill would thwart corporate tax moves Tax avoidance – The Irish inversion  "Congress should enact legislation immediately . . . to shut down this abuse of our tax system," Mr Lew wrote in the letter to Mr Camp, which was obtained by the Financial Times on Tuesday. "What we need as a nation is a new sense of economic patriotism, where we all rise and fall together," Mr Lew wrote.  The Obama administration proposal, which was backed by powerful liberal Democrats such as Carl and Sandy Levin, as well as Ron Wyden, chairman of the Senate finance committee, would clamp down on tax inversions by raising the threshold of foreign share ownership required for such a deal to be structured from 20 per cent to 50 per cent.  Read MoreUS drug firms seek inversion deals to evade taxes  By having it take effect starting in May 2014, the US Treasury and congressional Democrats hope to avoid a further increase in companies structuring such agreements over the next few months. However, retroactive application of tax changes is relatively unusual, and its use to reduce the number of inversion transactions has received criticism from business groups, conservative tax economists, and Republican lawmakers.  Mr Lew's letter on Tuesday was first reported by the Wall Street Journal. It follows plans revealed this week by Mylan, the US generic drugmaker, to move to the Netherlands through its purchase of Abbott Laboratories's European generics drugs unit.  The decision by Heather Bresch, Mylan's chief executive and the daughter of Joe Manchin, the Democratic senator from West Virginia, came just before it emerged that Walgreen, the US drugstore chain, is also considering an inversion to Switzerland as it weighs the purchase of the rest of Alliance Boots.
Treasury Secretary Jack Lew has urged Congress to crack down on merger deals in which companies seek to relocate overseas to cut US taxes.