UKIP leader Nigel Farage has insisted he did not "bottle it" by deciding not to run in the by-election to replace MP Patrick Mercer.
Mr Mercer stood down ahead of being banned from Parliament for six months over a "cash-for-questions" scandal.
But Mr Farage said he would not contest the Newark constituency, arguing he would look like an "opportunist".
Tory MP Ben Wallace called him a "chicken", while Labour's Rachel Reeves said he "didn't want to fight".
There had been speculation Mr Farage would be his party's candidate in the Nottinghamshire seat, following Mr Mercer's resignation on Tuesday.
But the UKIP leader announced on Wednesday that he would not be taking part.
Asked to respond to Conservative and Labour claims that he had "bottled it", Mr Farage said his opponents "could say what they like".
"A wise general does not fight every battle, he chooses his battles," he told the BBC. "I am not going to be parachuted into a constituency where I would be seen as opportunist."
Fighting Newark, an area he had no connection with, would be tantamount to "deserting" UKIP activists campaigning to try to come top in the European elections, he said.
He promised UKIP would "throw the kitchen sink" at winning the seat, held by the Conservatives with a majority of 16,152 at the 2010 general election.
Mr Farage said: "I think I have shown some courage over the years. I have helped take this party from nothing into a position where last Sunday it was leading the opinion polls in a national election."
He added: "I'm a fighter. I'm a warrior. But you have to pick your battles in life."
But Conservative defence minister Anna Soubry, a Nottinghamshire MP, said Mr Farage was "frit" - a word meaning frightened, which was used by Margaret Thatcher.
Fellow Tory MP Ben Wallace said: "That faint clucking you can here [sic] in the distance is Nigel Farage... chicken!"
For Labour, shadow work and pensions secretary Rachel Reeves said she was "not surprised" as Mr Farage "bottles it when there is a real contest".
"When he has a chance to prove that people want him as a Member of Parliament, he backs away. He's done it again in Newark."
No date has yet been set for the Newark contest. The Conservatives have selected Robert Jenrick to contest the by-election while Labour - which held the seat between 1997 and 2001 - has chosen Michael Payne to fight the seat. The Liberal Democrats are still to make a selection.
A nationwide poll for ITV by ComRes puts UKIP in first place among voters who say they will definitely take part in European elections on 22 May.
It gives the party 38%, while Labour is in second place on 27%, the Conservatives third on 18% and the Lib Dems fourth on 8%. ComRes interviewed 2,052 British adults between 25 and 27 April.
Meanwhile, Mr Farage has distanced himself from a UKIP donor after he claimed that there is "no such thing" as rape within marriage and that "lust, not love" determines homosexual relationships.
Demetri Marchessini, a Greek shipping and investment tycoon who has given £15,000 to the party, made the comments in an interview with Channel 4 News.
Asked whether UKIP should be accepting money from someone with such views, Mr Farage told the same programme "possibly not" but that it was a matter for the party's treasurer.