The five largest lenders in the UK accounted for 82% of all mortgage lending activity in 2009, according to the CML.
The concentration of gross lending among the top five largest firms has significantly intensified since the advent of the credit crunch, rising from 64% in 2007.
The CML said that further consolidation in the market would ensure the current trend continues into the long term.
The top five comprised of Lloyds Banking Group, Santander, Nationwide, Barclays and RBS.
Lloyds claimed top spot, taking 28% market share of all outstanding mortgage balances at £346bn and 24.1% market share of gross mortgage lending in 2009 at £34.7bn.
However, this was less than half of its 2008 total of £78bn of gross mortgage lending.
Santander came in second with 13.5% of all outstanding balances worth £166.8bn, up from £159.2bn in 2008. It increased its gross mortgage lending share to 18.4% from 13.9% in 2008 at £26.4bn, but this was notably down on 2008's total of £35.2bn.
Nationwide held steady in third place, with little change on 2008. It recorded an 11.2% share of the market worth £138.9bn, while its share of gross mortgage lending share rose from 11.6% to 17.8% in 2009 though the value of mortgages fell from £29.5bn to £25.5bn.
Barclays and RBS held joint fourth place in market share for outstanding mortgage balances, with 7.1% market share each at £87.9bn.
However, Barclays slipped to fifth place on gross mortgage lending with 9.9% market share, lending £14.2bn in 2009.
RBS took Barclays' place at fourth for gross mortgage lending, with 12.3% market share up from 7.4% in 2008, lending £17.6bn in 2009 down from £18.7bn in 2008.
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