As Greece made a successful return to the bond market this week – an important milestone for its troubled economy, new details emerged about the relationship between Prime Minister Alexis Tsipras and his former finance minister Yanis Varoufakis at the height of the crisis.

“When we got to the point of reading what he presented as his plan B it was so vague, it wasn’t worth the trouble of even talking about.

In an exclusive interview with the Guardian Tsipras opened up about the dramatic events of 2015 as Greece grappled in its negotiations with the troika, and the version of those events Varoufakis has presented ever since.

Admitting to having made “big mistakes” since Syriza’s election in January 2015, the 42-year-old PM told Athens correspondent Helena Smith that one of his biggest may have been “the choice of people in key posts”.

When asked if this referred to appointing Varoufakis, Tsipras responded saying that while his former finmin was the correct choice for an initial strategy of “collision politics”, Varoufakis as the architect of what became known as ‘plan B’ – a plan to supply emergency funding to the Greek financial system and return to the drachma – left the PM unimpressed.

“Perhaps the moment will come when certain truths are told…” said Tsipras. “When we got to the point of reading what he presented as his plan B it was so vague, it wasn’t worth the trouble of even talking about. It was simply weak and ineffective.”

After that withering put-down Tsipras went on to say that while the Syriza government’s original strategy was one of collision politics with the troika – “in line with our mandate” – leaving the Euro and the EU was never a possibility, even when Greece appeared to be on the brink of defaulting on its bailout loans.

“Leave Europe and go where … to another galaxy?” he tells Smith. “Greece is an integral part of Europe. Without it, what would Europe look like? It would lose an important part of its history and its heritage.”

Meanwhile, within hours of the article’s publication, Varoufakis was hitting back. Writing on his much-followed blog, he said:

“Given that I presented my plans to Mr Tsipras for deterring the troika’s aggression and responding to a potential impasse (and any move by the troika to evict Greece from the eurozone) before we won the election of January 2015, and I was chosen by him as finance minister (one presumes) on the basis of their merit, his answer reflects a deep incoherence.”

“Either I was the right choice to spearhead the ‘collision’ with the troika…because my plans were convincing, or my plans were not convincing and, thus, I was the wrong choice as his first finance minister.”

Twitter: @HelenaSmithGDN