A landslide Labour general election victory would lead to an "elective dictatorship", Lady Thatcher has warned.
The former Conservative prime minister said the country's future as an independent nation lay "in the balance" - but said it was not too late to do something about it.
Lady Thatcher's views coincide with a warning from Conservative leader William Hague that a huge Labour majority would be "dangerous" for the country.
"I have nothing in principle against large Commons majorities," Lady Thatcher writes in The Daily Telegraph.
"I applaud strong government, but not overweening government sustained by cronies, ciphers and a personality cult.
"I very much fear that, if Mr Blair is returned with a large majority, these already detectable tendencies will grow unchecked."
She continued: "A quarter of a century ago, the great Lord Hailsham ... warned of what he termed an 'elective dictatorship'. It is not too late to vote to prevent that."
Lady Thatcher defended her party's concentration on the issue of the euro.
She wrote: "Mr Blair has accused the Conservative Party at this election of being 'obsessed' with the euro. There must, though, be something profoundly wrong with a Prime Minister who is simply not concerned at all about his country's freedom and independence."
Mr Blair, she claimed, was "committed to policies that will lead to the progressive extinction of Britain as an independent nation state".
Meanwhile senior Tories today denied "cracks" were opening up in their election campaign amid reports that pro-Europeans in the party were preparing to round on the leadership if they go down to a heavy defeat.
Shadow environment secretary Mr Archie Norman insisted that the party could still triumph on June 7th, despite its failure so far to make any inroads into Labour's big opinion poll lead.
"I didn't come into politics to lose. I came to be in government," he told Sky News.
"I think there are going to be huge surprises on polling night. I do believe we stand every chance of winning. There are still a lot of people undecided. We are fighting to win."
Last night however Mr Hague appeared for the first time to concede the possibility of a Tory defeat when he warned of the dangers of another Labour landslide.
PA