MPs are expected to vote later on whether to put the precise time and date of Brexit into law.
The government wants to specify that the UK will leave the EU at 23:00 GMT on 29 March 2019 in its EU Withdrawal Bill, which is currently going through Parliament.
The plans have been criticised by opposition parties and some Tory MPs.
But a compromise amendment giving more flexibility is expected to help the government see off a rebellion.
March 2019 is already when the UK is due to leave the European Union – two years after Theresa May formally gave notice of Brexit.
The prime minister’s announcement that this would be enshrined in law was attacked by Labour as a “gimmick” and some Conservative MPs warned it could tie the government’s hands if negotiations dragged on longer than planned.
Ministers do not want another defeat on their key EU Withdrawal Bill – having already been outvoted last week on a different matter – and are expected to support the proposed compromise, tabled by Conservative MPs including former Cabinet minister Oliver Letwin.
The new amendment seeks to allow the government to change the “exit day” through further legislation, if the negotiations are continuing.
Labour described it as a “humiliating cave-in” by the government.
The EU Withdrawal Bill is a key part of the government’s Brexit strategy. It aims to end the supremacy of EU law, which would be copied onto the UK statute book in order to ensure a smooth transition on Brexit day.
It is facing hundreds of attempts to change its wording by MPs but only one – to give Parliament a guaranteed vote on the final Brexit deal – has been passed by the Commons so far.