Labour must start to make contingency plans for coalition talks with the Liberal Democrats after the next election, including being prepared to offer major cabinet posts to Nick Clegg's party, Lord Adonis, one of Ed Miliband's closest policy advisers, has said.

He also says a full-blown coalition is "massively preferable" to running a minority government, adding that he believes the bulk of the Liberal Democrat membership are ideologically closer to Vince Cable than Nick Clegg.

Adonis, who was transport secretary in the last Labour government and was intimately involved in the failed 2010 coalition talks between Labour and the Lib Dems, accused Clegg of misleading his party about those negotiations. But he said it was not right for Labour to tell the Liberal Democrats whether Clegg would have to stand down as a precondition of a coalition, something that some senior figures have suggested.

He stresses Labour is currently on course for victory, but it has to learn from the big mistake of the post-2010 negotiations, where no preparation for talks with the Liberal Democrats had been made. "Preparing for coalition negotiations properly," he insists, is "part and parcel of being serious about power, this is not defeatism at all."

Speaking to the Guardian in the week he published his book about the 2010 coalition talks, Five Days in May, and ahead of chairing a Progress conference on Saturday on how Labour could secure the next election, he said it was "stark staringly" obvious that a hung parliament was the most likely outcome in 2010. But Labour had no plans in place, partly because key figures such as Lord Mandelson had assumed Labour would lose outright.

He suggests Labour should be prepared to follow continental practice and be willing to offer the leader of Liberal Democrats a major office of state, such as home or foreign secretary.

He said: "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. 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."

Those preparations, he said, should include building good personal relations between the key leaders on both sides, a serious analysis of Liberal Democrat policies, a preparedness to be generous in terms of the allocation of posts and to work out what those posts may be. Adonis was one of the five-strong Labour team negotiating with Clegg's party after 2010.

He said: "I do not think Gordon Brown had any serious conversation with any member of his team about coalition negotiations until after the election. Yet any study of the electoral system showed it was stark staring obvious that a hung parliament was probable."

The peer, who now advises Miliband on industrial policy, asserts: "If the only way of forming a majority is to have a coalition, that in my view is massively preferable to a minority government; but if you're gonna have a coalition it has got to be for real and that means a serious joint programme and a serious joint sharing of the senior posts. 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.

"The big mistake that Nick Clegg made was to go into coalition but not into government, he didn't take any big position in government either for himself or for his colleagues. Continental practice is almost invariably for the leader of the second party to take a big post, I'm mystified as to why he didn't.

"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. There are seven big public service departments; he didn't get one of them for the Lib Dems."

He said the only thing left for Clegg's party to do was to "issue a periodic veto". He said the Liberal Democrats were in a near existential crisis.

Adonis also challenged the constitutional doctrine advanced by Clegg at the last election that the minority party has a duty to speak first to the party with the largest number of votes and seats, a doctrine that ensured in 2010 Clegg would speak first to the Conservatives.

He said: "That is not a doctrine which applies in any other democracy which has coalitions. The current Swedish government for instance is a coalition of the second largest party and other parties."

He denied his call for Labour to prepare for a post-2015 coalition would be unpopular in the party, saying: "The same cast of mind that goes hell for leather to get an overall majority is the one that also prepares for other eventualities." Describing Clegg as economically neo-liberal and culturally European, he insists it was Clegg's personal leadership that led his party to want to form a coalition with the Conservatives. But he denies he would want Clegg removed as the price of a coalition deal.

He said: "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.

The centre of gravity of the Lib Dems is to the left, he asserts, "which is why Nick had to invent new constitutional doctrines and straightforwardly mislead his party as he did after the election on states of negotiation with Labour in order to persuade them to go in with the Tories.

He said: "It came out very strongly in those negotiations in 2010 that Nick disagreed with his own party on key policy issues, including tuition fees and his own economic strategy. 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."