May had asked for maximum leeway to negotiate with Brussels on Brexit.
Lawmakers voted 309 to 305 to change the government's bill so that it guarantees Parliament will get a "meaningful vote" on the final Brexit deal at the end of negotiations in 2019.
The pro-European members of opposition parties were joined by rebel lawmakers from the governing Conservative Party to require that any final deal to withdraw from European Union be submitted to Parliament - as legislation - before it can be put into effect, the Guardian reported.
May had presented the argument that going through such formal approval would add yet another hurdle to Brexit.
Conservative rebels feared that without a specific, legal, guarantee of a vote, the Parliament might find itself being bypassed at the last minute. (ANI)