April 17 (Reuters) - The pound could take a big hit if Keir Starmer resigns as Britain's prime minister following a row over the security vetting of Peter Mandelson, his former ambassador to the United States.

Starmer on Friday faced renewed calls from his political opponents to resign after it emerged that Mandelson failed security vetting but was still allowed to take up the job. Senior minister Darren Jones told LBC that Starmer would update parliament on Monday.

The renewed controversy comes ahead of Welsh and Scottish parliament elections and local English elections on May 7, in which Starmer's Labour Party is expected to perform very poorly. The growing pressure has weighed on the pound, with GBP/USD dropping to threaten 1.35 and GBP/EUR falling to a four-day low of 1.1468 on Friday.

If Starmer were to leave Downing Street, cable could drop towards 1.30, a level it last threatened in November.
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(Robert Howard is a Reuters market analyst. The views expressed are his own)