Prime Minister David Cameron stood firm on Wednesday against eurosceptic members of his own party who criticised a plan to keep Britain in the European Union as a watered down "thin gruel" of broken promises.
The deal, presented by European Council President Donald Tusk on Tuesday, has so far received a warm response from European capitals which must approve it.
But at home, it risks reopening a longstanding wound in Cameron's Conservative Party, beset by division over Europe for decades.
Bringing the package to parliament, the prime minister faced more than two hours of sometimes hostile questioning, mostly from established eurosceptic ruling party members who had already written off Cameron's renegotiation as a waste of time.
The first round of what is likely to be a months-long battle was less heated in parliament than in the mostly eurosceptic British press, which trashed the deal as a "farce", a "joke" or a "delusion".