Nationwide says London house prices are 5% above 2007 levels, while in the rest of the UK they are 9% lower

The gap between house prices in London and the rest of the UK is now the biggest it has ever been, with homes in the capital more expensive than ever before, according to figures from the UK's largest building society.

Nationwide's latest house price index showed the average price of a UK home rose to £168,941 in June, an increase of more than £1,000 a month on December's figure of £162,262. Prices have risen by more than 4% in the first half of 2013 with demand being fuelled by cheap credit and improvements in wider economic conditions.

However, the society acknowledged that the headline figures masked "significant regional variation", and said the gap between house prices in London and the rest of the UK was the widest it had ever been, in both cash and percentage terms.

Its data, based on properties valued by the building society for mortgages, put the average price of a London property at £318,214, more than before the 2007-08 credit crisis and almost twice the average for the rest of the UK once the city is stripped out. The premium on London prices is £150,615.

While across the UK as a whole prices are around 9% lower than in the autumn of 2007, Naionwide said that in London they are 5% above that level. In contrast, in Northern Ireland homes are worth 52% less than they were in 2007.

Jonathan Hopper, managing director of property search consultants Garrington said: "For prices in the capital to be 5% above their 2007 peak is nothing short of incredible. The London property market is an extraordinary microcosm. It has effectively broken free of the rest of the UK and is operating in its own stratosphere.

"While its jobs market has been resilient, the primary driver of the growth has been strong overseas demand."

Countrywide, the society said prices were up by 0.3% in June, and that the annual rate of inflation had increased to 1.9% – the highest level since September 2010. The index has not shown a monthly price fall since the autumn and annual figures have been in positive territory for the whole of 2013.

Nationwide's chief UK economist, Robert Gardner, said a number of factors were likely to be behind the recent acceleration in prices. "Demand for homes has been supported by further modest gains in employment, as well as an improvement in the availability and a reduction in the cost of credit, partly as a result of policy measures such as the Funding for Lending Scheme," he said.

"Signs of a modest improvement in wider economic conditions may also be playing a role in boosting buyer sentiment."

While demand was picking up, Gardner said there were few signs of a significant improvement in the supply of houses, with construction data showing that building activity had fallen from already depressed levels.

Howard Archer from IHS Global Insight said: "We believe that a strong upward move in house prices is unlikely for now, given a still challenging and uncertain economic environment – despite recent signs of improvement. In particular, very weak earnings growth argues against a substantial rise in prices, while consumer confidence is still limited.

"Housing market activity is also currently still limited compared to long-term norms despite the recent pick-up in activity."

Separate figures from property website Zoopla showed a 32% increase in the number of £1m homes in Britain, with 10 neighbourhoods in London boasting average property prices in seven figures.