Dozens were injured when high winds caused part of a stage to collapse at a dance music festival near the Spanish city of Valencia
"One ERG source says 40 have switched to back deal, 40 undecided but it's for each individual to declare before we're sure," Sam Coates said.
To get her deal approved, May needs 75 lawmakers to come over - dozens of rebels in her Conservative Party, some opposition Labour Party lawmakers and the Northern Irish Democratic Unionist Party (DUP), which props up her minority government.