Other top brokers, including Willis, are also considering whether to allow their staff to postpone payments until after 6 April when the top rate of income tax falls to 45%.
Just days after the investment bank Goldman Sachs was forced to back down on its own plans to defer bonuses, it has emerged that Aon has put mechanisms in place to delay payments for its employees so they pay income tax at 45%. The move allows an executive expecting a £200,000 payout to save £10,000 in tax.
Goldman's U-turn came after criticism from the Bank of England governor, Sir Mervyn King, and pressure from politicians. He told the Treasury select committee: "I find it a bit depressing that people who earn so much seem to think it's even more exciting to kind of adjust the timing of it to get the benefit of a lower tax rate which they will benefit from in the long run to a very great extent."
Goldman then announced it was no longer considering whether it should defer bonuses from 2009, 2010 and 2011, which were due to be handed to staff in the coming weeks, before the top rate fall.
Many City employers with staff who pay the top rate of tax are known to have considered whether to defer payments into the new tax year but experts believe some of them have decided against such a move following the row over the lack of tax paid by coffee chain Starbucks in the UK.
Aon is among a number of major insurers to press ahead with deferring bonus payments while Willis said it was still considering whether to do likewise. "Willis takes this matter very seriously and we are reviewing it," said a spokesman for the insurer, which employs around 3,000 staff in the UK.
The Aon bonuses relate to the 2012 financial year and were due to be paid in March. They will now be paid in April after the beginning of the new tax year.
A spokesperson said: "Qualifying UK employees were given the option of deferring payment of their 2012 bonuses from March to April 2013. Approximately 250 employees, 4% of the UK employee base, elected this option."
Aon, which sponsors the Manchester United football team shirts, owns several businesses in the UK including insurer Benfield and HR consultants Hewitt. It moved its headquarters from Chicago to London last year so it could be nearer to the Lloyd's of London insurance market.
The relocation of Aon's HQ triggered forecasts of millions of pounds of extra revenue for the exchequer so the latest move may prove to be an embarrassment for the chancellor, George Osborne, who had refused to publicly criticise Goldman over tax.
A treasury minister, Sajid Javid, though, spoke to top officials at the bank shortly after King's comments to MPs.
The Bank of England governor told the committee: "It would be rather clumsy and rather lacking in care and attention to how other people might react … in the long run, financial institutions, like all large institutions, depend on good will from the rest of society, they can't just exist on their own."
David Cameron had criticised the tax affairs of the comedian Jimmy Carr publicly in June but the government had been slower to lambast companies in such a public way.
The coalition has warned for the past year that it would reduce the 50% top rate of tax imposed by Labour which came into effect in 2010.
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