Speaking during a lively debate with Ukip leader Nigel Farage on LBC this afternoon, the former deputy prime minister and In campaign spokesperson, Lord Heseltine, said: "I think that there will come a time when Britain will join [the euro]."
"I do think that if the European venture succeeds, and I believe it will, warts and all, then it will come a time when Britain will reconsider that decision."
When Farage expressed his "delight" that Britain didn't join the common currency, Heseltine hit back saying he needn't worry because it wouldn't happen in his lifetime.
While he'd previously stated this position towards the end of 2014, the euro has since attracted renewed criticism due to its role in the Greek debt crisis.
The Leave campaign seized on Heseltine's remarks, saying that they show staying in the EU carries great risks.
Ian Duncan Smith, a former work and pensions secretary who is in favour of Brexit, said: "Today they boasted that staying in the EU means deeper integration — financial as well as political."
He continued: "Not least it will mean the UK joining the euro — a currency that is currently on the rocks."