A political row over the Irish government’s failure to force the former chairman of a state-owned bank to meet his debt obligations is threatening to undermine support for the European Union’s Lisbon Treaty on the eve of a crucial referendum.

Although the most recent polls, released last weekend, gave the Yes vote a comfortable lead, a senior Labour party official said on Thursday that recent financial scandals could persuade a large proportion of undecided voters to vote No. “It is a worrying trend that has been continuing for the best part of the week,” said Joe Costello, the party’s European affairs spokesman.