The firm goes on to say thatU.S.shareholders “could recognize a capital gain on the receipt” of the shares.

Aon says that those who own Aon stock outright could be subject to capital gains taxes, whereas those with restricted or deferral arrangements may not be subject to capital gains taxes.

The filing, which is a copy of a letter sent to shareholders, advises shareholders that they should contact a tax advisor to understand how their holdings will be treated.

Aon says it has hired PricewaterhouseCoopers “to provide assistance to colleagues that have additional questions about how this transition will work and what it means for your holdings.”

According to a piece in The New York Times, the move from theUnited States may not have sat well with founder Patrick Ryan. The story cites people “familiar with his thinking” as saying he “was not doing cartwheels” after the announcement.

Ryan, along with his wife Shirley, own close to 4 percent of Aon's outstanding shares, close to 13 million.

Ryan has not commented on the move.

For Aon, the firm says the move would give it access to $300 million of excess capital that it holds internationally on its balance sheets. The firm says it would also increase its cash flows “through a significant reduction” in its global effective tax rate. Such a move would leave it to “remain competitive with certain global competitors.” It will also reduce its global tax rate over the long term.

Aon says it is not abandoning theUnited States, leaving itsU.S.headquarters inChicagowhere it signed a 15 year lease on theAonCenterin the downtown area. The firm says in addition to making that commitment, it will move an additional 750 jobs into the building and plans to add 1,000 new positions to the company's roster as demand for services grow.

In a letter, Greg Case, Aon's president and chief executive officer says the firm will continue to show its commitment toChicago with a commitment of $15 million in charitable giving.

The firm says around 20 executives will be relocating to London, including Case who will receive a foreign service allowance of $135,000, housing allowance of $336,000, cost of living allowance of $90,000 and one-time relocation allowance of $80,000. Case receives an annual base salary of $1.5 million.

In another filing, Aon said Jan Kalff is retiring from the board. Aon says his decision was “solely for personal reasons” and had nothing to do with any “disagreements.”

When the board voted on the move, reportedly three board members abstained from voting on the move. Kalff was not one of those board members named in a New York Times story on the move.