Britain's richest 1% have accumulated as much wealth as the poorest 55% of the population put together, according to the latest official analysis of who owns the nation's £9.5tn of property, pensions and financial assets.

In figures that also lay bare the extent of inequality across the north-south divide, the Office for National Statistics said household wealth in the south-east had been rising five times as fast as across the whole country.

The average wealth of households in the southeast had surged to £309,000 at the end of 2012, up 30% since the first wealth report published by the ONS covering 2006-8 – while the average rise in England was only 6%.

But wealth in the north-east had fallen, the only region where it did so, to an average of just under £143,000. In Scotland the figure was £165,500.

The data also shows that one in nine households have second homes or rental properties and one in 14 sport a personalised number plate on their car.

Northern regions lost out after a dramatic rise in stock market values that was grabbed mostly by households in the south east, the ONS figures show.

The situation is likely to have worsened following an 18% surge in house prices over the past year in the south-east and even higher at the top end of the market.

A rush to save among richer households as the recession deepened boosted the nation's total wealth and ensured Britain's long-established financial inequality remained in place, with the top 10% laying claim to 44% of household wealth – while the poorest half of the country had only 9%.

Rachael Orr, Oxfam's head of poverty in the UK said the figures were a "shocking chapter in a tale of two Britains".

The charity recently reported that five billionaire families controlled the same wealth as 20% of the population. "It is further evidence of increasing inequality at a time when five rich families have the same wealth as 12 million people," she said. "We need our politicians to grasp the nettle and make the narrowing gap between the richest and poorest a top priority. It cannot be right that in Britain today a small elite are getting richer and richer while millions are struggling to make ends meet."

Duncan Exley, director of the Equality Trust, said: "The grotesque concentration of wealth in the hands of a tiny minority is fracturing our society, weakening our economy and giving disproportionate power to the richest. Unless policymakers adopt a clear goal of reducing the gap between the richest and the rest, they will have to govern an increasingly dysfunctional nation."The report comes after French economist Thomas Piketty has ignited international debate about inequality by documenting the rapid accumulation of assets by the top 1% over the four decades since the 1970s.

Britain's top 1% saw their share of wealth increase slightly in the four years before 2012, grabbing the same share as 54.9% of the population, up from 54.2% in 2008/10.

But the Treasury said the report showed that wealth inequality had remained the same throughout the six years up to 2012 while income inequality had declined to levels last seen in 1986. A spokesperson said the government's efforts to protect the poorest during the recession had worked.

"The effects of the Great Recession are still being felt which is why we have taken continued action to help hardworking people by cutting income tax and freezing fuel duty.

"And we want to help more people to save for their future or own their own home which is why we are giving people more flexibility over their pensions and introducing Help to Buy. At the same time we have introduced new higher rates of stamp duty on the most expensive homes and done more than any previous government to crack down on tax evasion and avoidance in order to ensure that everyone pays their fair share in tax."

Critics of the wealth report said it failed to capture the huge diversion of wealth to offshore tax havens, which account for trillions of pounds worth of savings.

A series of investigations into offshore tax havens have documented the success of their banks in attracting a steady rise in the savings and financial assets of the richest 1%.

There was also a clear disparity between women and men over who owns the most homes, pensions, cars and stocks and shares. The average value of men's total pension wealth was nearly twice as high as women's in 2010/12 – £63,000 compared with £34,800.

The power of the grey pound is highlighted in the report by several measures, including one showing that couples without children, where one person is over and the other under the state pension age, have the highest total wealth at £607,800, up from £452,000 in 2006.