MICHAEL Fingleton (72) cannot be legally forced to hand back the controversial €1m payment as he was contractually entitled to it.
He was paid a total package of €2.4m in 2008, including a salary of €893,000 and almost €450,000 in holiday pay.
The €1m sum was paid as "an incentive" to keep Mr Fingleton employed as chief executive by the building society during a time of market turmoil.
Mr Fingleton had stepped down from Nationwide's board in January 2008 as required by the institution's rules, having reached the age of 70.
The building society had considered changing its rules to remove the age bar, but this was rejected at its annual general meeting. However, Mr Fingleton found this decision "unacceptable".
Negotiations began to ensure he remained with the society and it was agreed that his salary for 2008 would be increased by 10pc from €812,000 to €893,000 with an extra payment of €1m.
The bonus "was not to be linked to performance or any further assessment".
Mr Fingleton also has the largest pension pot in the country at €28m. He benefited from a range of extraordinarily "generous" top-up arrangements and he upped his pension scheme just before the credit crunch began in 2007, preserving its value.