Labour leader Keir Starmer is being investigated over potential rule-breaches on earnings and gifts.
He is subject to the probe by Westminster’s standards watchdog.
The investigation by the Parliamentary Commissioner for Standards was confirmed on the UK Parliament’s website.
The case was opened against the Labour leader on June 8.
During a visit to Wakefield in West Yorkshire, Starmer insisted he is confident he has not broken the MPs’ code of conduct, saying “there’s no problem here”.
He told broadcasters that the allegations were not a surprise, adding: “My office is dealing with it and will be replying in due course.”
Asked if he was sure he had done nothing wrong, he said: “Absolutely confident, there’s no problem here.”
A spokesman for the Labour leader said: “Keir Starmer takes his declaration responsibilities very seriously and has already apologised for the fact that administrative errors in his office have led to a small number of late declarations.
“The Standards Commissioner has asked for more information which we are happy to provide.”
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