June 25 (Reuters) - Confidence in Labour Prime Minister Keir Starmer, and the pound, could be shaken if rebel lawmakers within his party derail his plan to reform Britain's welfare system.

Starmer has vowed to press on with the reforms despite opposition by more than 100 of his MPs.

If a House of Commons vote provisionally scheduled for July 1 goes ahead and torpedoes the government's welfare plan, it would be a major blow for Starmer a year after Labour won a landslide majority in the general election.

It would also cast doubt on whether Starmer will still be Labour leader at the next general election, expected in 2029.

CFTC data on FX positioning, due on Friday, will show if the net GBP long rose in the week ended June 24, after falling 17% to 42,857 contracts in the prior week. GBP/USD scaled a 41-month peak of 1.3648 on June 24.

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