Major banks have moved to shield their margins on home lending by slicing base rates on some of their most popular savings accounts marketed to depositors.As previously reported in Banking Day, most lenders' funding costs have come under pressure as the bank bill swap rate and offshore wholesale funding benchmarks have risen this year.While many small lenders such as Bank of Queensland, ME Bank, AMP Bank and ING began raising rates on owner-occupier and investment mortgages last month to retrieve margins, the major banks have resisted taking similar action fearing they may trigger a ferocious political backlash.However, Westpac and CBA have decided to defend their net interest margins by hacking further into the returns they make to depositors.CBA this week chopped the base rate on its NetBank Saver account by 30 basis points after Westpac last week also slashed the return on its eSaver product by the same amount.Three of the major banks - CBA, Westpac and ANZ - are now offering annual rates of only 0.5 per cent on their online savings accounts.Repricing moves from NAB and ANZ are also expected in the coming weeks as they each try to preserve their respective net interest margins, which were each reported at around 1.9 per cent at the end of March this year.Westpac and CBA are protecting much thicker margins of around 2.1 per cent each.Although the deposit rate cuts might help to delay home loan repricings in the near term, financial products research provider Mozo suspects that the major banks will also hike mortgage rates some time in the next eight weeks."We might be some way off from home loan rate increases from the Big Four banks, but I wouldn't be surprised if they moved in the next six to eight weeks," said Peter Marshall, Mozo's product data manager."As lending volumes and house prices start to decline, the major banks will probably look to protect profitability as much as they can by addressing their net interest margins."In addition to mortgage rate increases, Marshall believes the banks could also lower some other deposit rates.The disconnect between the market price of money and the Reserve Bank's official cash rate has widened throughout 2018.The spread between 90-day bank bill swaps and the cash rate climbed to 57 basis points on Tuesday, its highest level in more than seven years.Historically, a persistent spread of this magnitude was often a signal that an official rate rise of 25 basis points was imminent.Marshall's view is that product repricings will continue to occur even if the RBA keeps the cash rate on hold. "We're not going to see the banks move in line with the cash rate," he said. "I think it's going to become a difficult time for the banks' public relations teams and their consumers as well."