Tony Blair has promised to oppose plans by the European Parliament to stop countries such as the UK from opting out of the Working Time Directives.
Working hours have prompted fierce debate.
He said the UK would have enough EU support to block a measure he called "wrong" and "completely misguided".
At his first news conference since the election, Mr Blair said he had "no intention" of abolishing the UK opt-out from the 48-hour working week limit.
European MPs voted this week to phase out the opt-out over three years.
Mr Blair said: "We've got to have a fundamental debate in Europe about the direction Europe wants to go...
"The competition that Europe's economies face from the emerging economies of the world mean that we simply cannot afford to give up our flexibility."
Under the current system - used more in the UK than elsewhere - individuals can opt out if they want to work longer hours.
To block the European Parliament's plans, the UK would need to recruit other countries' support to retain the clause under qualified majority voting in the EU council of ministers.
Questioned about a UK referendum on the European Union constitution, Mr Blair said no decision had been taken on the likely timing of this.
But he added that even if the French gave a "no" vote on 29 May, "we will have a referendum on the constitution in any event".
But all member states need to ratify the constitution for it to become law.