Next PM will be leader who commands confidence of of the Commons, not necessarily leader of largest party, ex-cabinet secretary reminds Cameron

Gus O’Donnell, the former cabinet secretary, has reminded David Cameron that he signed up to a Whitehall document that says the next prime minister will be the leader who commands the confidence of MPs, regardless of whether their party has won the election.

In a sign of concerns in Whitehall that constitutional conventions should be followed if voters elect the most balanced parliament in nearly a century, Lord O’Donnell indicated that the leader of the largest party does not automatically become prime minister.

O’Donnell spoke out after the Cameron accused Ed Miliband of preparing a “con trick” to enter No 10 with the support of the SNP.

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In an intensification of his warnings about the dangers of a post-election deal between Labour and the SNP, the prime minister said such an arrangement would raise huge questions about the credibility of that government.

He told the Today programme on BBC Radio 4 on Wednesday: “I think it is a con trick because Ed Miliband is saying I’m not going to do a deal with the SNP, I won’t have an agreement with the SNP. But actually he knows the only way he can become prime minister is with the backing of SNP MPs.”

But O’Donnell told the same programme shortly before Cameron’s appearance that the key qualification to enter No 10 was the ability to command the confidence of the House of Commons. In a Guardian interview in March, the former cabinet secretary said that the polls at that time suggested that “a Lab-Lib coalition with a deal with the SNP” looked like a feasible scenario. The polls once again suggest that the combined seats of the three parties would achieve a parliamentary majority.

O’Donnell told Today: “We live in a parliamentary democracy. The rules are very clear and they are laid out in the Cabinet Manual and that says the ability of government to command the confidence of the elected House of Commons is central to its authority to govern.”

He later reminded the prime minister that he signed up to the Cabinet Manual. He told the Daily Politics on BBC2: “One thing I should say about the Cabinet Manual. People keep saying it is my Cabinet Manual. It is the government’s Cabinet Manual. It is the cabinet’s Cabinet Manual in particular, and the preface is there signed by the prime minister, David Cameron.”

The former cabinet secretary, who also appeared on Sky News on Wednesday, indicated that he would take to the airwaves again after the polls close on Thursday night to ensure that the guidelines in the Cabinet Manual were respected.

He said: “I’ll be in London at various studios trying to ensure that those interpretations that come out of the exit poll at one minute past 10 are actually in line with what’s in the Cabinet Manual, and people aren’t saying: ‘Oh well, I think I’ve got a chance because I got this number of seats or whatever’.”

O’Donnell published a draft of the chapter of the Cabinet Manual on the formation of a coalition government in early 2010 because he expected voters to elect a hung parliament. The draft was endorsed by Gordon Brown as prime minister. The manual was fully published after the 2010 election with a foreword by Cameron on behalf of the coalition. This means the manual has the backing of the three main parties.

In his Guardian interview in March, O’Donnell said it was wrong to assume that the leader of the largest party should automatically become prime minister. His remarks were then aimed at Nick Clegg, who has always said that the Lib Dems would talk to the leader of the largest party in the first place in coalition negotiations.

O’Donnell said: “The one thing we need to be aware of is people thinking that what Nick Clegg said last time constituted an iron law that only the biggest party, somehow defined either by seats or votes, gets to have the first say. That is not true.” Asked whether the prime minister is the person who can command the Commons he replied: “Precisely.”

The former cabinet secretary is unlikely to be taking such a high-profile role on the airwaves without the agreement of Sir Jeremy Heywood, his successor as cabinet secretary.

Britain could elect its most balanced parliament since 1923 when Ramsay MacDonald, the Labour leader, became prime minister with the arms-length support of Herbert Asquith’s Liberals even though Stanley Baldwin’s Conservatives had came first in terms of number of seats.