Nigel Farage said he had "done no wrongdoing" and followed parliamentary rules after reports he failed to declare support from ally George Cottrell before entering Parliament.
The Sunday Times said that support included security, social media staff working on Farage's online content and use of a property near Buckingham Palace in the year before his election.
Josh Babarinde, a Liberal Democrat MP, has asked the parliamentary standards commissioner to investigate, arguing Farage must explain who funded help linked to his political rise.
Farage's team says the alleged in-kind benefits did not need registering because they were received before he became Clacton MP and were not political, echoing his defense in a separate probe over a £5 million gift.
Cottrell, 32, is a longtime Farage ally who admitted a US wire-fraud count in 2017; Farage is also considering legal action against the Sunday Times and calls the report an establishment attack on Reform UK.