Across the UK, households had an average of around £7 a week more in discretionary income in March than they did a year ago, according to Asda's latest income tracker, compiled by the Centre for Economics and Business Research (Cebr).Discretionary income is money left over after taxes and bills are paid to spend on "luxuries" such as holidays, sports, cinema tickets, dining out and savings.This 4.3% annual jump in spending power is the most rapid rate of annual growth seen since autumn 2012, and it means that across the country families had around £170 a week to pay for non-essentials last month.However, Northern Ireland's rate of £80 a week is much, much lower than other parts of the UK and is over half the amount available in other regions.In Scotland, an average of £177 is spare each week while in Wales, it sits at £149.All regions have seen an annual growth in their spending power, but the report said that "spiralling" living costs in London mean that the increase it is seeing lags behind the rest of the UK by comparison.Household spending power in the North East has grown by 5.5% annually to the first quarter of this year, while in London it has edged up by just 2.8% year-on-year.The gap between the level of discretionary incomes in London and those in the rest of the UK has narrowed slightly, from nearly 37% a year ago to just under 36%.But despite the pick-up, the average discretionary income in the North East is still half of that of London households, where it now stands at around £231 a week.The report said that as well as high living costs, discretionary income growth in London is more "subdued" due to a strong dependence on the financial and business services sector, which continues to see declines in pay.Rob Harbron, senior economist at Cebr, said: "Robust economic growth is expected to continue, which means it is likely that discretionary incomes will continue to rise over the coming months."A Treasury spokesman said: "Today's strong numbers from Asda are welcome news for hard working people in all parts of the country."Coming alongside last week's news of record employment, these numbers provide further evidence that the Government's long term economic plan is working and bringing greater economic security."These remain difficult times for families facing pressures on their budgets, and much work needs still to be done to build a resilient economy. But today's news supports the argument the Government has made all along that the only way to see rising living standards is to grow the economy."