In its latest annual PensionWatch survey, the body said almost £33 million had been handed out to senior executives in this way over the last year.
The chief executive of taxpayer-backed Lloyds Banking Group, Antonio Horta-Osorio, is among those taking cash instead of contributions, receiving £549,390 last year. The banking boss also received a £731,000 pension contribution into a defined benefit scheme and £18,170 in a defined contribution plan.
Earlier this year, Lloyds was criticised for placing a freeze on the pensions of 32,000 workers to create a £1 billion saving.
TUC General Secretary Frances O'Grady said: "Most workers, if they're fortunate enough still to be in a company pension scheme, will be retiring on a lot less than they would have a generation ago.
"Not so for company directors who will still be looked after very handsomely in their retirement.
"There may have been a move away from more generous defined benefit schemes for top directors in recent years, but this change certainly does not mean that they are losing out. Unlike employees, who have seen the value of their pensions slashed, company bosses are now getting huge cash payouts on top of their substantial salaries."
The PensionsWatch survey shows that almost two-thirds (64 per cent) of FTSE-100 senior executives now receive money for at least part of their company's contribution towards their retirement. The average amount received by senior directors in this way was £149,493 (17 per cent of their salary). The report also reveals that for defined contribution pension schemes, the average contribution rate for executives is 12 per cent, while employees receive just 6.6 per cent. Only a few FTSE-100 companies have the same contribution level for both executives and employees.
A spokesman for Lloyds said Mr Horta-Osario's cash pension is to compensate him for the pension arrangements he had to give up when he moved from Santander.
He claimed that this pension is worth "significantly less" than the CEO's previous pension and is linked to share price targets. The spokesman added: "In line with many other UK companies, Lloyds Banking Group reviewed its pension arrangements in order to ensure that it continues to offer a competitive and sustainable pension to all its employees.
"We have concluded it would be appropriate to provide a one-off lump sum payment to employees who are currently members of defined benefit schemes. Unlike a number of companies, the Group has decided that it will keep its defined benefit pension schemes open to future accrual but has changed the pensionable pay cap."