Walking away or posting the keys through the letterbox is called 'abandonment' and will not end your agreement. Your agreement with the landlord will continue even though you've left and the landlord can continue to charge you rent, so you're likely to build up rent arrears: if your agreement is fixed term, you can be charged rent until the term ends if your agreement is periodic, you can be held liable until the agreement could have been ended by giving proper notice. The landlord can apply for a court order to make you pay what you owe. The court will decide whether you should have to pay your landlord the money or not. The landlord may have to show that s/he has tried to find another tenant for the property but hasn't been able to do so. If the landlord has managed to let out the property they can't claim rent from you after the date the new tenant moved in. If the landlord doesn't make any effort to let out the property the court is likely to reduce the amount of money you will have to pay.