Jeremy Corbyn has avoided saying whether he will use his party's whips to order Labour MPs to vote in favour of triggering Article 50.
The Labour leader was asked three times by Sophy Ridge on Sky News whether he would use his party's whips and refused to give a decisive answer.
Asked what his party's policy on leaving the EU is, Mr Corbyn said: "We accept the result of the referendum. It's clear.
"It's the result of a democratic decision that was made, parliament must reflect that. We therefore have to negotiate a way out. Therefore Articler 50 gives us the responsibility and the opportunity to negotiate."
He said such negotiations must include an effective trading relationship with Europe and that regulations on environment and European arrest warrants are continued in some form.
"So we will not block Article 50," he said, "we will however amend the bill to make demands on market access, in order to make demands on regulation."
However, Mr Corbyn avoided saying whether he was prepared to use the party whips to get his MPs to support Article 50.
"What we're saying is we will not block Article 50," Mr Corbyn replied.
Asked again whether he will use the whips, he said: "The Government is being challenge in the Supreme Court on whether or not it should consult Parliament. We have forced them into a position where they have said Parliament will finally get a vote on this issue.
"We will not block Article 50 but we will seek to amend the bill. We are consulting on amendments to the bill which will include issues on rights, on protections and on market access."
Pushed for an answer a third time, he said: "I will ask all Labour MPs to respect the result of the referendum and allow Article 50 to be opened so we start that two year, probably longer, period of negotiation."
Mr Corbyn's position puts him at odds with many of his party's MPs, who have statements declaring their intent to vote against Article 50 pinned to their Twitter profiles.
