Labour leader Jeremy Corbyn has hinted he would attempt to block the parliament's vote on Article 50 if key principles were not met.
He said the UK must have "access" to Europe's single market, guarantees need to be made on safeguarding consumers and the environment, and that there should be no watering down of worker's rights.
Corbyn outlined his sticking points in the Sunday Mirror, but said he was not trying to reverse the result of the EU referendum.
He said: "These must be the basis of the negotiations. And it doesn't necessarily cause a delay.
"The court has thrown a big spanner in the works by saying parliament must be consulted. We accept the result of the referendum. We are not challenging the referendum. We are not calling for a second referendum.
"We're calling for market access for British industry to Europe."
It is unclear what Corbyn was referring to by the UK continuing to have "access" to the European single market. Any country which is not subject of sanctions can trade with members of the single market. However, the terms of such access are dependant on trade deals between the country and single market members.
However, that stance was reportedly confused somewhat when a source close to Corbyn told Sky News that Labour's support for triggering Article 50 was "unconditional".
The Labour source said: "We won't be blocking Article 50. Our support for invoking Article 50 is unconditional".
Instead his party "would seek to amend or influence the government's negotiating terms" after voting for the move.
Hilary Benn, who now chairs a powerful parliamentary committee on Brexit, also confirmed Labour would not vote down the Article 50 process.
However, as the prime minister's Conservative majority is only 12 seats, Labour and rebel Conservative MPs could collectively vote to block the Article 50 from being triggered.
As a result, Theresa May might have to call an early general election and secure a larger Conservative majority, if she was to force her will.
Corbyn added: "If the government calls an election we're ready for it. We have the members, the organisation and the enthusiasm. We welcome the challenge.
"It would give us the chance to put before the British people an alternative economic strategy for this country."