It follows Monday’s high-profile Dispatches programme on Channel 4 on 'cash for access', in which MPs Jack Straw and Sir Malcolm Rifkind discussed the possibility of working as consultant lobbyists.
In response to the programme, the CIPR wrote to White asking to clarify whether MPs and members of the House of Lords who also undertook such work would need to register. Speaking to The Guardian, White confirmed that they would as long as they were paid to do so and were registered to pay VAT.
White told the newspaper: "Although the Lobbying Act is not specific on this point, my view is that serving MPs and members of the House of Lords in the context of their normal duties would not be required to register.
"But if they undertook activities outside their normal duties which might be defined by the act as consultant lobbying, and where other exemptions such as VAT registration did not apply, they would be required to register."
Alastair McCapra, CIPR chief executive, called it "a helpful clarification" but said the body was "concerned about the VAT exemption and [has] been all along".
"Parliament has legislated to make it a criminal offence to act as a consultant lobbyist without registering. This law must be applied to anyone who meets the definition in the [Lobbying] Act.
"However, it is conceivable that lobbyists handling controversial issues or working for controversial clients will be able to avoid registering by not being VAT registered. This could include a number of ex-MPs after the general election in May, who set themselves up as lobbyists to cash in on their contacts, but who will not have to register for VAT until their taxable earnings reach £81,000.
"We understand the intention to protect small businesses, but the principle would be better applied if the Cabinet Office heeded our calls to drop the policy of flat fee for registration and introduced a graduated fee structure based on business size, meaning sole traders who pay VAT could join the register without incurring fees that could cripple their business."
PRWeek contacted the Cabinet Office for comment but had no reply at the time of publication.
The statutory lobbying register is due to go live on 27 March.
Yesterday Labour failed to gain enough support in the House of Commons for a ban on MPs holding paid directorships or consultancies. The party's motion was defeated by 287 votes to 219.
Angela Eagle, Labour's shadow leader of the Commons, said the current situation was "untenable", although her Conservative counterpart William Hague said Labour's motion was "unclear".