The deputy Data Protection Registrar, Francis Aldhouse, yesterday announced his office's intention to carry out the inquiries.
'We will be taking this issue up with all leading banks because we are not quite sure what the situation is,' he said.
'We think the straightforward approach is to get in touch with them and see what their practice is.'
He described having information about political affiliations as 'potentially sensitive' and suggested customers were entitled to an explanation.
However, Mr Aldhouse accepted NatWest's claim that it never solicited political information - or other details which added more to a personal profile.
The bank said it merely made a note if such information cropped up in conversation.
Mr Aldhouse said: 'We certainly don't want to set hares running or suggest there is anything sinister about it.
'We have no reason to suggest that the situation is anything other than that expressed by the NatWest bank. If it were anything more than that we would have to consider our position.'
A spokesman for NatWest said it had permission from the Data Protection Registrar to hold such information.
Labour's City affairs spokesman Alistair Darling, who has tabled a Commons motion calling for regulation of banking services, welcomed the move by Mr Aldhouse, which followed a Daily Express report yesterday.
'I hope he makes it public because the public want to know what information is held about them,' Mr Darling said.
Leading article, page 17