Ed Miliband should not be sucked into a pact with the Liberal Democrats and instead show the courage to rule on a minority government, Unite General Secretary Len McCluskey said.
"Labour, I hope, win the next election outright, but if they are the biggest party then my view is Ed should have the courage of his convictions and govern on a minority government," , Mr McCluskey said in an interview with the BBC's Newsnight programme.
"My view is that Ed shouldn't be sucked into a Lib/Lab pact he should have the courage of his convictions if we are the largest party, he should govern.
"And he should challenge those coalition parties to bring him down if necessary and go back to the people so that there's a stark alternative."
More top news
A report published by the Boundary Commission for England details plans to reduce the number of MPs by scrapping 50 constituencies.
The slump will spark scrutiny, especially as statistics show a rise in racially or religiously motivated offences following the EU vote.
The 53-year-old was killed shortly after leaving her home in Mosta, Prime Minister Joseph Muscat said.