France is expected to demand the removal of the post of British European commissioner as a price for a long Brexit delay, leaving Britain without a seat at the top table of Brussels decision-making for the first time since 1973.

Senior EU sources say the French president, Emmanuel Macron, is likely to seek to entrench the UK’s reduced status in the EU at the leaders’ Brexit summit on Wednesday evening.

The British government will have to agree to losing its commissioner but it is unlikely the prime minister will put up a fight when she is presented with the terms of an extension.

“There are different legal means not to have a [British] commissioner,” said an EU diplomat, suggesting one way could be the British government declining to nominate anyone. “The question of non-participation could depend also on the British government.”

Leaders are expected to reject Theresa May’s appeal for a limited extension to 30 June, but the EU27 are split between those who support a Brexit delay until 31 December year, and those who think the end of March 2020 would be safest.

The European commission president, Jean-Claude Juncker, is said to favour an extension to March 2020, to allow the maximum feasible amount of time for Westminster to find a way forward and “kick the can down the road”, a senior diplomat said.

France has been an outspoken critic of allowing the UK to extend its EU membership without a clear plan, suggesting that Britain’s crisis should not be allowed to take the bloc “hostage”.

But Paris is now expected to turn its focus on the conditions of a further Brexit delay, including the removal of Britain’s seat in the European commission, currently filled by Sir Julian King, who has the security portfolio.

“The commission will demand this; it is logical,” claimed an EU diplomat, adding that a British commissioner could not take a seat “if the term of the mandate is short”.

Under a provision of the Lisbon treaty, Brussels is able to reduce the number of commissioners from its current 28, under which each member state is represented in the EU’s executive branch.

France’s expected move will not receive the automatic support of other EU capitals who would be concerned about the loosening of the tie between the number of member states and commissioners.

Britain’s first commissioners at the time of its accession to the then European economic community in 1973 were Christopher Soames, a son-in-law of Winston Churchill, and George Thomson, who had previously sat in Harold Wilson’s cabinet as secretary of state for Commonwealth affairs.

Senior diplomats have suggested that France would also be more comfortable with any extension being limited to the end of the year due to concerns in Paris over Britain becoming a difficult member state.

But under the terms of its extended membership, the UK will be asked to sign up to an “enhanced duty of sincere cooperation” with the EU, in reference to a responsibility to keep out of the bloc’s long-term planning.

France wants the summit communique to add in language stating that the “UK will not obstruct decisions taken at 27”.

The EU, on the advice of Juncker, will also follow the precedent set following the formation of the Eurozone, under which countries that have not adopted the euro are left out of meetings.

In this case, only the UK would be outside the room on discussions over long-term planning, such as the appointment of the new president of the European commission and the seven-year multi-annual financial framework (MFF), setting out the top lines of the budget for the next commission term.

The UK would be expected to refrain from using its veto on the MFF but senior diplomats said there would be a way around the problem if the British government went “rogue”. “We can make the decisions together at 27 and formalise it once the UK has gone,” said a source.

A senior EU diplomat said the risk of Britain turning into a disruptive member state had been overblown by the Conservative MP Jacob Rees-Mogg and some EU capitals for their own purposes.

A draft summit communique, obtained by the Guardian, and to be agreed by the EU27’s leaders on Wednesday, assumes in return for an extension a “commitment by the United Kingdom to act in a constructive and responsible manner throughout this unique period in accordance with the duty of sincere cooperation”.

It goes on to say the EU “expects the United Kingdom to fulfil this treaty obligation in a manner that reflects its situation as a withdrawing member state”, while the length of the extension is left blank.

“To this effect, the United Kingdom shall facilitate the achievement of the union’s tasks and refrain from any measure which could jeopardise the attainment of the union’s objectives,” it adds.