April 21 (Reuters) - The lack of noise from Labour Party lawmakers in Britain's Parliament during Keir Starmer's 140-minute grilling over Peter Mandelson on Monday is ominous for the prime minister and the pound.

With Welsh Parliament, Scottish Parliament and English local elections looming on May 7, things could soon go from bad to worse for Starmer. Disastrous May election results for Labour could spur a leadership challenge against Starmer, which could hurt sterling on fear of what comes next should he be ousted.

Just as the pound plummeted during the ill-fated reign of Liz Truss after the Tories chose her to succeed Boris Johnson as PM in September 2022, so it might again if Starmer exits Downing Street and his successor scares the bond market.

Whereas a centrist such as Wes Streeting, Shabana Mahmood or John Healey could be well received by the gilt market and sterling should they enter Number 10, a soft-left successor like Angela Rayner, Andy Burnham or Ed Miliband might spook investors.

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(Robert Howard is a Reuters market analyst. The views expressed are his own)