The EU is due to meet on Monday to consider a Brexit extension until 31 January, with an option for the UK to leave earlier if a deal is ratified.
A draft text to be shown to ambassadors from the 27 member countries includes multiple possible dates for Brexit: 30 November, 31 December or 31 January.
There will also be a commitment that the Withdrawal Agreement cannot be renegotiated in future.
Unless the extension is approved, the UK will leave the EU on Thursday.
Under the proposed legal decision to extend the Brexit process, the EU would also retain the right to meet without the UK to consider future business during the extension.
The EU has so far agreed to an extension after Boris Johnson was forced by Parliament to request it, but has not specified the new deadline date.
If EU members approve the request for a three-month extension, Mr Johnson would have to accept it, under the terms of the law passed by MPs to prevent a no-deal Brexit.
Any extension to the Article 50 process is technically flexible, meaning that it can come to an end as soon as a deal is approved by both parties.
But by writing this flexibility into the legal decision to delay Brexit, EU leaders aim to underline their neutrality from the political debate in the UK.
A short delay could be seen as risking a no-deal Brexit by those who support remaining in the EU, while a long delay could be seen by Brexit supporters as attempting to prevent the UK from leaving.
As part of the extension, the EU is expected to say the UK has "an obligation to suggest a candidate" to represent it on the EU Commission.
Mr Johnson has previously said "under no circumstances" will he nominate anyone to take over from Julian King when the new commission takes office on 1 November, arguing that officials need to focus on the UK's future outside the EU.
'Look at all options'
The decision from Brussels is set to come as the UK debates how to use any potential extension to break the Parliamentary deadlock.
The prime minister will put forward a motion calling for a 12 December election on Monday, which needs the support of two-thirds of MPs to succeed.
But the Liberal Democrats and Scottish National Party want to see a bill introduced that enshrines a 9 December election in law, as long as the Brexit deadline is extended to 31 January.
Conservative MP James Cleverly dismissed this plan as a "gimmick" and Labour leader Jeremy Corbyn called the move a "stunt".
But a Downing Street source said the government would "look at all options" if its own election motion failed.
Unlike the government plan, the Lib Dem-SNP bill would only require a simple majority to pass.