Can a guarantor withdraw his guarantee before the loan has been repaid and what are the consequences of such action ?

Assuming the guarantee was enforceable in the first place (i.e. not procured through fraud or duress, guarantor had capacity to contract, etc.), and the guarantee does not provide that it can be cancelled (in cases involving lines of credit or similar arrangements there could be such a provision, but it's very unlikelywith an outstanding loan), the guarantor cannot cancel his/her guarantee without the approval of the creditor.  Hard to say what the consequences would be since it can't be done; if the debtor defaults, the guarantor will be liable on his/her guarantee, whether he/she has purported to cancel it or not.