George Osborne has publicly attacked key policies in Theresa Mays manifesto just days before the election as he pledged not to pull punches after quitting politics.
The former Tory chancellor, who was sacked by Mrs May when she became prime minister, called her social care proposals badly thought through.
He also said the Prime Ministers repledged immigration target could force family members to live apart and accused her of abandoning liberal politics.
The interventions, made in a BBC Radio Four interview on Saturday, were given not as a politician but as the editor of the Evening Standard, the London newspaper.
However they will infuriate Mrs Mays team who view Mr Osborne with suspicion after a series of critical headlines in the newspaper he now runs.
The attacks saw Mr Osborne compared to Margaret Thatcher and Edward Heath, former Tory prime ministers who were a constant thorn in the side of their successors.
Mr Osborne was unexpectedly sacked from the Cabinet when Mrs May took office last July and has since decided not to seek re-election as an MP.
Asked about Mrs Mays social care plans - which saw the party scrap universal winter fuel payments and the pensions triple lock - Mr Osborne was critical.
"They were clearly badly thought through because the Prime Minister herself decided to rethink them, he said, referencing the decision to announce a care cost cap after a backlash.
On the decision to keep a promise to get net migration below 100,000 a year, Mr Osborne said the Tories needed to give details on how it would be delivered.
He said: "Which section of industry is not going to have the labour that it currently needs? Which families arent going to be able to be reunited with other members of their family abroad? Which universities arent going to have overseas students?
Now, if the Conservative Government can answer those questions, all well and good. If they cant, the Evening Standard is going to go on asking the question. We will also be as ferocious in asking questions of the Labour Party.
Mr Osborne also criticised the direction of the Conservative Party, saying: Both Theresa May and Jeremy Corbyn are offering in very different ways a retreat from international liberalism and globalisation.
he former Tory chancellor repeatedly defended his views by saying it was his duty to scrutinise politicians now he was a newspaper editor rather than an MP.
I have to call it as I see it as an editor. Of course everyone knows I was a Conservative MP for 16 years, I was a member of the Conservative cabinet and I know many of the people in the Conservative Government, he said.
But its also my responsibility now as the editor of the paper to interpret whats going on in politics and the rest of life for my readers. So Im not pulling punches because I think I would be doing readers a disservice.
However the decision to go public with his criticisms just two weeks before an election and with the Tory lead slipping in the polls is likely to anger Mrs Mays allies.
Mr Osborne also shrugged off claims that the immigration target had been adopted by his own Government and repeatedly missed. He added he was having great fun as editor.
I haven't seen a prime minister be as unwilling to have their opinions challenged as she has been for a while.
