​It depends, said the English Court of Appeal in Harvey v Dunbar Assets plc, [2013] EWCA Civ 952. Harvey and three others agreed to guarantee the obligations of Vision Development Ashbrooke Ltd to its bank. Harvey disputed liability when the bank came calling to enforce the guarantee, arguing that the signature of one of the other parties was a forgery. The trial judge found that Harvey was liable on the guarantee, notwithstanding the alleged forgery.

The Court of Appeal reversed. There is no 'absolute rule, or enshrined principle' that if one party to a guarantee has not signed it the others are not bound: where a party signs a guarantee in circumstances where other contemplated security has not been obtained, whether that party has actually assumed liability is really a question of interpretation in light of the factual matrix. Where, however, a guarantee is entered into by A as a joint and several guarantor on the basis of a representation that the guarantee will be executed by B, the liability of A is conditional on the execution of the guarantee by B, unless the guarantee contains express language to the effect that A (or B) will be bound whether or not the other person actually signs. On the facts, this guarantee was clearly a single composite document, prepared for signature by four parties identified as joint and several guarantors. It was likely that the signature of all four was 'an essential precondition' for liability, but the inquiry doesn't end there: was there anything in the contract to exclude the general rule and bind a party even in the absence of other signatures? The definition of 'the Guarantor' as 'every person liable under this Deed' suggested that one actually had to be -- that is, have become -- liable, not just be 'potentially liable' (as the bank argued). Other provisions tended to support the conclusion that execution by all parties was a precondition to the liability of any one of them.

http://www.bailii.org/ew/cases/EWCA/Civ/2013/952.html