LONDON, Aug. 3 -- Richard Tice is under investigation by Britain's Parliamentary Standards Commissioner. He wants to be precise about why.

The Reform UK deputy leader and Member of Parliament said Monday that the inquiry stems not from the donation investigation that London police began examining last month, but from a complaint lodged by what he called an anti-Semitic and anti-Israel lobby group. The distinction, in his framing, is everything.

In late July, British police opened a criminal probe into Reform UK donations, examining approximately $333,000 channeled to the party from a company called Britain Means Business, of which Tice is a director. The inquiry centers on whether those contributions were disclosed and classified proper...