David Trimble has accused the Alliance Party of engaging in election "stunts" after it referred a phony election leaflet in circulation to the police.
David Trimble said he had no idea who produced the leaflet
Alliance said the leaflet from a group calling itself Concerned Citizens for a Shared Future may breach election law.
The leaflet, printed in Alliance colours, urged its supporters to vote for the UUP in seats the DUP may win.
Mr Trimble said he was "astonished at the behaviour of the Alliance Party complaining about leaflets".
He told BBC Radio Ulster's Inside Politics that he had "no idea" who had produced them.
"I've seen the content of them, and someone no doubt thinks they're being helpful to us," he added.
"I've got my doubts as to whether that's in fact helpful to us.
"But people are entitled to do this, and to complain to the police about it looks a bit like a stunt."
Alliance leader David Ford said the leaflet was party of a "dirty tricks campaign" which was "clearly designed to confuse" his party's supporters.
"The publishers hide behind a PO Box number and urge Alliance supporters to vote for the Ulster Unionists, wrongly claiming that the UUP supports Alliance ideals such as a shared future," he said.
"There is also a graph that gives the wrong figures for party support at the assembly election."
He said the leaflet had been distributed in at least six constituencies, adding that the party had made a formal complaint to the Electoral Commission and had asked the police to investigate.