"Councillor Johnson, cabinet member for housing, said it was part of an ongoing campaign.
He said: "We want to do everything we can to stop tenants getting into rent arrears."
He said the message was "deliberately hard-hitting" but not intended to be insulting.
He added: "It's a very serious issue and the real issue of this double-sided postcard was to say if you are in difficulty give us a call.
"It would give me no pleasure to evict people in the new year because they're behind on their rent."
He said that eviction proceedings begin when a tenant is £250 in arrears.
He said he believed the card, which was sent to about 17,000 properties, cost about £2,000 to produce, which "pales into insignificance" against the rent owed.
He added: "I think we've got to take all reasonable steps possible to make sure people pay their rent."
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