Downing Street is prepared to look at other options should its plans for an election fail, a Number 10 source says.
The government will table a motion calling for a 12 December election on Monday, but this needs support from two-thirds of MPs.
However the Liberal Democrats and SNP want to introduce a bill that enshrines a 9 December poll in law subject to a Brexit extension to 31 January.
Earlier, Conservative James Cleverly dismissed this plan as a “gimmick”.
The Lib Dem-SNP bill amends the Fixed-term Parliaments Act 2011 to include the date of 9 December as the next general election and this would come into force if the EU decides to extend the Brexit process to a date no earlier than 31 January.
BBC political correspondent Jessica Parker said it was significant as this method would only require a simple majority than the the two-thirds required to back the prime minister’s motion for an election on 12 December.
A number 10 source said: “Tomorrow MPs will vote on an election on 12 December so we can get a new Parliament.
“If Labour oppose being held to account by the people yet again, then we will look at all options to get Brexit done including ideas similar to that proposed by other opposition parties.”
Speaking to the BBC’s Andrew Marr Show, Mr Cleverly said of the Lib Dem-SNP bill: “It’s clearly a gimmick.
“Their bill moves the election date by three days, takes the withdrawal agreement completely off the table.”
He said the government had put forward proposals for a general election first.
“What we’re not going to do is, we’re not going to listen to two parties who have explicitly said they want to stop Brexit from happening,” he said.
“We’re not going to be complicit in them stopping Brexit from happening.”
Mr Cleverly also said he was “cynical” as the bill would be amendable – so MPs can suggest changes to it.
Asked about possible amendments to her proposed bill, Ms Swinson told the BBC’s Andrew Marr Show that although any bill in Parliament can be amended, “the intention is very much that this is a simple bill that can be passed through Parliament quickly”.
She said the “time pressure” involved in securing a poll before their desired 31 January deadline meant the party would not pursue amendments such as votes for 16-year-olds to the bill.