Lloyds Banking Group said it will lose 15,000 staff by 2014 as part of new chief executive Antonio Horta-Osorio's strategic review, while HSBC said it will cut 700 roles.
The unions said they were "flabbergasted" by the day's job losses and accused the banks of finding "any excuse" to cut them.
Mr Horta-Osorio, who took up the top post in March, was unrepentant and insisted the cuts had to be made for the good of the bank and UK economy.
The latest cull at Lloyds, which owns brands including Bank of Scotland, Halifax and Scottish Widows, will bring total job losses at the 41% state-owned bank to nearly 45,000 since it was formed in 2009 when Lloyds TSB and HBOS merged.
Portuguese-born Mr Horta-Osorio pledged to create a more "agile" organisation, with the majority of job cuts in middle management and back office roles.
David Fleming, Unite national officer, said the Lloyds review will cause "deep distress and anxiety".
He said: "Astonishingly one in eight roles will be lost over the next three years. This review is merely another box-ticking exercise to give this bank - which has already, since its creation two years ago, cut over 27,000 staff - an excuse to sack more employees."
Mr Horta-Osorio also announced plans to reduce the company's international presence from 30 countries to less than 15 by 2014.
He also pledged to revitalise the Halifax brand and will keep the Scottish Widows insurance arm.
The group said it has repaid a hefty chunk off the liquidity support from the Government and Bank of England - which was set up in 2008 as a lifeline for banks battling to raise finance during the credit crunch.
Lloyds said it had reduced the facilities from £97 billion at December 31 to £37 billion and was on course to repay completely by October 2012.
Addressing concerns about job losses, Mr Horta-Osorio said: "It's important to note that these are roles not people. We have a strong record of minimising redundancies."
But he added: "We have to do this. This bank has lost money, it's losing money this year on an after-tax basis.
"We have to get this bank back on to its feet to support the UK economy and we have to pay taxpayers' money back."
Lloyds Banking Group has 104,000 full-time staff worldwide.
In the UK, it has main offices in London, Cardiff, Edinburgh and Belfast among others. Mr Horta-Osorio added that some of the 15,000 job losses will come from plans to downsize overseas, while his plans to "revitalise" the Halifax brand will see it launch a new advertising campaign in September.
The chief executive would not go into detail but did say the bank's 650 branches will start opening on Saturdays in a bid to improve customer relations.
He pledged to keep branch numbers at the same levels, outside its agreement with the EU to sell 632 outlets, and would not offshore further UK permanent roles.
Mr Horta-Osorio said Lloyds, as the largest retail and commercial bank in Britain, was committed to the UK.
He said the UK economy and Lloyds were "inextricably linked" and the review was based on a cautious economic outlook.
Mr Horta-Osorio won over investors with his vision for the business as Lloyds' shares lifted by as much as 8% at one stage on the FTSE 100 Index.
Bruce Packard, financials analyst at Seymour Pierce Research, said the overhaul "looks sensible".
HSBC said it was reshaping its wealth management business before Retail Distribution Review rules are introduced in January 2013.
The new law will mean UK banks can no longer offer financial advice for free, so HSBC expects demand for the service to decline. However, the bank will still have 1,500 advisers across the UK.
David Fleming said the union was "flabbergasted" by HSBC's staff cuts and accused the bank of attempting to "bury its own bad news" on the same day as Lloyds announced its job losses.
He said: "The workforce being hit by these extensive cuts today were in no way responsible for the banking crisis, yet it is these staff, many of which are low-paid, who are having to pay for the bank's recovery."
HSBC last month put its US branch network and credit cards business under review as part of a multibillion-dollar cost-cutting plan.
In the UK, it said it will evaluate its position once the final decision of the Independent Commission on Banking is published, which could force it to ringfence its retail business.
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