Plans to put big EU treaty changes to a UK referendum are a "massive advance", William Hague has insisted amid criticism from Tory MPs.
Defending the EU Bill ahead of an MPs' debate on Tuesday, he said it would ensure any future government could not "wriggle out of a referendum".
Some Tories say the bill is "smoke and mirrors" and does not go far enough.
Labour's Yvette Cooper said it was a "dog's dinner" which was all about trying to keep Tory Eurosceptics happy.
The European Union Bill would ensure "significant" EU treaties must be approved by a referendum of UK voters, with the same rule in place for major changes to existing treaties.
The plans for a "referendum lock" were first mentioned by David Cameron in 2009 after he had ruled out a referendum on the Lisbon Treaty following its ratification by all 27 EU member states, something which irritated Eurosceptic Tory MPs.
Under the bill, ministers could rule out a referendum if they judged that the transfer of power to Brussels was "not significant".
They would have two months from any change to decide, with their decision open to challenge via a judicial review.
Eurosceptics attacked the EU Bill during its last Commons reading in December. It will return to the Commons on Tuesday and there have been reports of a possible Conservative backbench rebellion.
Veteran Tory Eurosceptic Bill Cash has put down a series of amendments to the bill.
On his blog on Sunday, Tory backbencher Douglas Carswell called for a referendum on the EU itself and described the bill as "smoke and mirrors".
"This bogus EU Bill is no substitute for the referendum we were promised. Nothing in this bill will cause the permanent British representatives in Brussels, who really decide Europe policy, to change course," he wrote.
Writing in the Sunday Telegraph, Mr Hague recognised that some felt the bill did not go far enough, or had loopholes.
But he insisted: "The truth is that only in a few minor areas does it give the ministers of the day any discretion at all about the calling of a referendum - and then only if they can persuade Parliament and the courts that they are right.
"When it becomes an act this will be the strongest defence of national democracy put in place anywhere in Europe. It is a massive advance for national democracy."
Prime Minister David Cameron also told BBC Radio 5 live on Sunday: "We said in the manifesto we will make sure that if politicians try to take powers from Westminster to Brussels you, the British people, will be given a referendum.
"If you read the bill it is really clear. This is a manifesto pledge delivered and I am really proud of that."
But shadow foreign secretary Yvette Cooper said: "This bill is about failed Tory party management, not the issues that matter for Britain in Europe.
"Instead of concentrating on things like growth, exports or cross-border crime, William Hague is wasting time trying and failing to keep his Eurosceptics happy.
"Even worse, the bill is so badly drafted and contradictory that it could lead to a lawyers' paradise where important decisions happen in court rather than Parliament."