David Martin, who has served as one of Scotland’s representatives in the EU parliament since 1984, previously said the Brexit vote had forced him to reconsider his opposition to Scottish independence.
With his time as an MEP possibly coming to an end this month, he said he was “not sure I want to be part” of the UK outside the EU.
“I campaigned vigorously in 2014 for Scotland to remain part of the UK, but - and Brexit is only part of those process - I see England, and I have to use the narrow term England, becoming more and more inward looking and unrealistic about its place in the world,” Mr Martin told Scotland on Sunday.
“I’m still not clear whether an independent Scotland would be the answer to that. Things can change quite quickly in politics, as we’ve been discovering.
“Whether there’s any road left for the United Kingdom, and whether an independent Scotland would genuinely be that much different, these are all still open questions for me.”
The veteran MEP welcomed the Labour Party’s moves towards a second referendum on EU membership, but claimed it would fall to the next generation of activists to make Jeremy Corbyn's party truly internationalist and campaign to rejoin the bloc.
“When I was first elected, the party position was to withdraw from the European Union and I campaigned inside the party to change that position,” Mr Martin said.
“I wrote a Fabian pamphlet in 1987 called ‘Bringing Common Sense to the Common Market’, and Neil Kinnock rather generously said in his autobiography that he saw that as the basis for Labour changing its position on the European Union.
“The direction of travel for the next 20-30 years was more and more integrationist and pro-European. I think it’s a terrible mistake for a party of Labour’s history not to remain internationalist, not to remain in a close partnership with our neighbours, and at the moment, if we leave the EU, I don’t see Labour in any way wishing to reverse our withdrawal.
“I don’t actually see much internationalism in terms of how we’re going to stay engaged with the world community, so I think there’s a big fight to be had inside the party again to ensure we remain an outwards-looking internationalist party.”
Sounding a note of regret at the end of his career in front line politics, Mr Martin said: “As a mea culpa on behalf of people like myself, with the benefit of hindsight, we just did not do enough to explain the benefits of the European Union.
“Sometimes it was difficult to get that message across, but I am sure that more could have been done.
He added: “Like in the old song Big Yellow Taxi, you don’t know what you've got until it's gone.
“A lot of the younger members of the party, who grew up being able to cross frontiers without any hindrance, being able to study abroad, being European citizens without really giving much thought to it, are all waking up to what they’re losing, and I think the interesting thing is that even an organisation inside the Labour Party like Momentum, 70-80% of their membership supports staying in the EU.
“If any good has come out of the referendum and the withdrawal process, it’s that we now have generation of under 30s who are absolutely committed to the European project, and want to stay involved - and if they can’t stay involved, want to get re-involved as soon as possible.”