The recent downturn was less deep than previously thought while annual growth was stronger as the economy began to recover, according to revised official figures.

Changes to methodology used by the Office for National Statistics (ONS) show gross domestic product (GDP) shrank by as much as 6% in the depths of the recession, lower than the previous “peak-to-trough” estimate of 7.2%.

The revisions for 1997 to 2012 show the size of the economy on average 4%, or £50 billion, larger than previously thought each year. Estimates of growth have gone up by an average of just 0.1% per year to 2%.

But between 2007 and 2012, average growth has been revised higher by 0.5% a year, mainly as a result of faster growth in investment.

ONS chief economist Joe Grice said: “Despite the wide ranging improvements underpinning the new estimates, the broad picture of the economy has not changed much.

“Although the downturn was less deep than previously estimated and subsequent growth stronger, it remains the case that the UK experienced the deepest recession since ONS records began in 1948 and the subsequent recovery has been unusually slow.”

The ONS changes include new methodology to bring economic measures into line with international standards.

Among these are the inclusion of spending on research and development and weapons as investment as well as the introduction of spending on illegal activities such as drugs and prostitution in the calculation of nation’s income.

The revised figures showed pre-downturn growth in 2007 was 2.6%, lower than the previously thought 3.4%.

But the contraction in 2008 was 0.3%, not 0.8% as previously thought, and the decline for 2009 narrowed to 4.3%, from an earlier estimate of 5.2%. Growth in each of the three subsequent years was better than previously thought.

The data failed to provide a solution to the so-called “productivity puzzle” facing economists, with output per hour worked at the end of 2012 still around 12% below the projected path had pre-downturn rates of expansion been maintained.

The ONS said the length of the downturn remains unrevised. Further figures on the impact of the changes to date will be published later this month.

It comes after the latest report from the World Economic Forum lifted the UK one place to ninth in its international competitive ratings.