Senior Conservatives are prepared to challenge new rules which could allow Gordon Brown to remain as prime minister for up to a week after a defeat on Thursday, to try and form a coalition with the Liberal Democrats.

Shadow ministers have criticised the guidelines drawn up under the supervision of the cabinet secretary, Sir Gus O'Donnell. The Tories accused O'Donnell of overstepping his constitutional authority by giving his blessing to a delay in the return of parliament to allow talks on a coalition in the event of a close poll result.

David Cameron was not consulted on the changes which will mean that the Queen's Speech will not be held until 25 May – 19 days after the election. In 2005 the Queen's speech was held 12 days after polling day. MPs will return on 18 May to elect the Speaker, 12 days after the election. In 2005 there was a six-day gap.

Senior shadow ministers are making it clear that Cameron, who intends to lead a minority government if the Tories fail to win a parliamentary majority, is prepared to ignore the rules. These are designed to allow for a week of discussions between the party leaders on forming a coalition.

Senior Tories believe Cameron will require political momentum if he wins the largest number of votes but falls short of an outright majority. Allowing Brown to remain in office for a week could check that momentum.

The Queen does not summon a prime minister to resign; convention dictates that is for the prime minister to tender his or her resignation. Cameron indicated yesterday that he is prepared to challenge this convention.

The Tories fear Brown will use the new rules to follow the example of Edward Heath, who tried to hang on after the February 1974 election to broker a deal with the Liberal party. Heath, who won the most votes but secured four fewer seats than Labour, resigned four days after the election, after the talks broke down.

Brown, who could still win the largest number of seats while falling behind the Tories in the number of votes, could use the extra week to woo Nick Clegg.

One Tory frontbencher said: "The idea that a courtier like Sir Gus O'Donnell will decide this is straight out of the Victoria and Albert Museum. Sir Gus is damaged goods in our eyes. He is closely associated with Gordon Brown from his time in the Treasury." Another said: "This is a way of saying if we have a hung parliament why worry about politicians? It is mildly anti-constitutional."

Downing Street said last night that the new rules were designed to implement the recommendations of a report by the cross-party Commons modernisation committee in 2007. The report was accepted by the Speaker and by the government.

A Cabinet Office spokeswoman said: "The decision over when the house should return after an election is a matter for the government, not the cabinet secretary or the civil service."

The Tories remain suspicious of O'Donnell because he recently published a draft chapter of a new cabinet manual outlining the guidelines for a hung parliament. Shadow ministers noted that it was a senior civil servant who outlined the delayed convening of parliament before a formal announcement was made by No 10 on the day the election was called.

O'Donnell believes the new guidelines are an honest attempt to avoid the uncertainty that could follow a hung parliament. He has also guarded against an attempt by Brown to win over the Lib Dems by making major government announcements next week. The draft chapter says government "purdah" rules, which ban such announcements in an election campaign, would be extended to cover the period of negotiations.