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Sir Keir Starmer is being urged to scrap a law forcing people who live in mobile homes to hand over 10% of the profits when they sell up to the site owner.
The charge has been labelled "legal theft of equity" by campaigner Sonia McColl, one of an estimated 200,000 park homes residents across England.
She said most of the residents are retired and are selling up to pay for care, whereas the site owners are "making a profit on their profit".
Sonia handed in a petition at 10 Downing Street on Thursday, which 37,000 people have now signed up to.