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Only you and your employer knows the reason. Which to be honest, at a sudden £12k increase just before applying for ILR, appears to be an increases for exactly that reason, to meet the salary threshold for ILR. Stranger's on the Internet can't give good reasons as we don't know you or your employer. Also not a good idea to lie to HO either.Please any one suggest me what could be the reasons we can provide to home office for increment in my salary