Daniel Greenberg confirmed the probe on the commissioner's website, describing it as in relation to the "registration of interests under category one of the guide to the rules - employment and earnings".

No further detail was given, but Sky News understands it relates to speculation over an appearance the shadow minister for domestic violence and safeguarding made on TV where guests would usually be paid. However, she waived the fee - meaning there was nothing to be declared.

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A Labour spokesperson said: "It would be inappropriate to comment on an ongoing investigation."

Ms Phillips becomes the ninth person under investigation by the commissioner, with allegations against MPs ranging from damaging the reputation of the House through to using Commons' stationery.