Former foreign secretary’s stinging criticism of party after crushing defeat by Conservatives is likely to revive speculation of return to UK politics

David Miliband has said the Labour party has been sent “back to the classroom for the second time in five years” after its crushing defeat in the general election.

The former foreign secretary previously said “deep and honest thinking” was required after his brother, Ed, led Labour to its worst election defeat since 1983.

In comments that will add to speculation that he plans a return to British politics, Miliband said he was part of a Labour team that won elections rather than lost them. “I was in the backroom in the early 1990s when Labour in the UK figured out how to win elections rather than lose them,” he said. “As this month’s election in the UK showed, the electorate have recently sent us back to the classroom for the second time in five years.”

Miliband, who is president of the International Rescue Committee, made the comments at the Harvard Kennedy school graduation address. In the lecture the former Labour frontbencher also launched into a stinging criticism of David Cameron’s foreign policy and said Britain was politically “in retreat”.

“The UK is one of the most open and globally engaged cultures in the world; DfID (the Department for International Development) is a world leader in aid programming but politically we are in retreat,” he said. “The defence budget is going down; there is a referendum coming on whether to stay in the EU.”

Miliband expounded on his views on foreign policy and warned that international rivalries and deteriorating relations between powerful countries such as the US and Russia were hampering international peace.

He said Syria had “descended into hell”, and said its demise highlighted the danger of not constructing a proper foreign policy. “Syria is where all the problems, and the absence of solutions, of modern foreign policy come together – so much so that the human consequences are losing their capacity to shock,” he said.

“No, the truly shocking aspect of what is happening in Syria is the extraordinary absence of a political process to seek to solve the problem. Or even any impetus towards a political process. Instead, Syria is declining amid a desperate and dangerous silence; and its problems are being exported to its neighbours.

“Deteriorating relations between the US and Russia over Ukraine, the rivalry between Saudi Arabia and Iran, and rifts between the backers of Syria’s fragmented opposition have sapped hope that a meaningful political process can take place. And, of course, the high price of the mistakes of the west in Iraq and Afghanistan have sapped confidence that western powers will do anything other than make things worse, and drained support for anything other than hand-wringing. That is the explanation for the UK’s almost complete absence from the political as well as military battlefield.”

He said humanitarianism alone would not solve intractable conflicts and called for an international political solution.

Miliband said he supported French proposals, which would mean permanent members of the UN security council should agree to suspend their right of veto in cases of mass atrocity.