Former Labour leadership candidate Owen Smith, ex minister David Lammy, and shadow ministers Daniel Zeichner and Catherine West have all said they would oppose Article 50, despite Labour pledging not to do so.
The Liberal Democrats said they would formally oppose Article 50 at Westminster to avoid an un-democratic “stitch up”.
The party has long opposed leaving the EU without a further referendum on the terms of Brexit.
Leader Tim Farron told Today programme on Friday: “We would vote against Article 50 if our red line is not met.”
He continued: “Our red line is to respect the will of the people and that means they must have their say in a referendum in the terms of the deal.
“There will be a referendum at the end of this process so that nobody would have imposed upon them something they didn’t vote for.
“We believe what started with democracy last June, must not now end up with a stitch up with a deal imposed on the British people that nobody voted for.”
Yet with just eight MPs in the House of Commons, the party may need to rely upon its some 100 members in the House of Lords to influence the government’s plans.
But Farron explained: “The Lords will not get a vote and will only move amendments, so this will only be about the elected House of Commons saying to the British people: this is up to you.”
Dulwich Labour MP Helen Hayes said she would defy Labour whips unless the government promised a second referendum.
She told the BBC: “I had somebody in my surgery last week who was in tears because of Brexit and I see genuine distress amongst my constituents about what this path means.
“I would not be representing them if I voted to trigger Article 50 on the basis of no information from the government about the path that they would then take us on.”
The SNP’s 54 members in the Commons are yet to declare their voting intentions, but party leader Nicola Sturgeon has made clear she believes Brexit goes against the interests of the Scottish people.
Last week the High Court ruled the Westminster Parliament must be consulted about leaving the European Union.
Unless the higher Supreme Court overturns that judgement next month, a bill to invoke Article 50 is expected in the new year.
The Scottish government has appealed to the Supreme Court for to consider if the Holyrood Parliament should also be consulted.