Labour must draw up plans for a possible coalition with the Liberal Democrats if it fails to win the next general election outright, a senior peer said.
Lord Adonis said a detailed strategy was vital and should include proposals to grant senior ministerial posts to the junior partner's MPs.
The former transport secretary, who played a central role in Labour's failed bid to secure a deal with the Lib Dems in 2010, urged Ed Miliband not to repeat the mistake of being unprepared.
Labour must draw up plans for a possible coalition with the Liberal Democrats, says Lord Adonis
"We should prepare for coalition, not because we expect it to happen but because if we're responsible about governing the country then we have to prepare for serious eventualities," he told The Guardian.
"It was a big mistake before the last election, as I think all of the key players on the Labour side now realise, not to prepare properly for coalition negotiations.
"The same cast of mind that goes hell for leather to get an overall majority is the one that also prepares for other eventualities."
The peer said forming a coalition would be "massively preferable" to a minority government but said lessons should be learned from the difficulties faced by the present Tory/Lib Dem power-sharing deal.
"It has got to be for real and that means a serious joint programme and a serious joint sharing of the senior posts," he told the newspaper.
"Unless both sides have a real stake in the government, in terms of people and policy, any future coalition will go the way of this coalition and quite quickly become a fig leaf for the rule of the dominant party."
He said he was "mystified" that Nick Clegg had not insisted on taking one of the major offices of state such as foreign or home secretary, as was the practice in continental countries with regular coalition administrations.
"Not only did he maroon himself in the Cabinet Office with no departments and responsibility only for constitutional reform, which turned out to be the great black hole of Lib Dem hopes, but he didn't get any senior jobs for his colleagues either."
Lord Adonis suggested that the Liberal Democrats were generally much closer to Labour than Clegg.
"His coalition with David Cameron was an opportunity for him to sign up to policies that he really believed in rather than the party's policies that had been forced on him," he said - accusing the Deputy Prime Minister of misleading his own party over the negotiations.
But he cautioned however against any suggestion that a post-election deal would require Clegg to step aside as leader.
"I just think there's basic respect between parties and it is wrong if you're trying to tell a coalition partner what it's policies should be and who it's leader should be."