The annuity provider found people living in London saw a 48 per cent income drop once they were in retirement.
The average employment income in London was £20,300 while the average pension income was £12,000.
Those living in the South East had the highest pension income of £12,400 and this was the only region to receive more than £12,000.
Those in Wales saw the least drop with salaries totaling £17,500 and pension incomes totaling £11,600.
On a county basis, Buckinghamshire and Cambridgeshire were most badly affected with falls of 45 per cent and 44 per cent respectively.
The average amount for the UK was from a salary of £19,000 to a pension income of £11,600, a drop of 38 per cent.
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Andrew Megson, managing director of retirement at Partnership, said: "While people in retirement are likely to have fewer outgoings, it is still hard to imagine that anyone would not feel the pinch if they lost a third of their income overnight. Even if their pension is topped up by income from savings and investments or part-time work, it is still likely to be a shock."
Mr Megson said people who were forced to stop work earlier than they intended would be particularly affected.
He said: "In addition, many will have to stop working earlier than they intended because of health and lifestyle issues so may miss out on the opportunity to make additional financial provision and they are likely to find the gap is even more profound."

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