May's government on Monday avoided a humiliating defeat in Parliament when it narrowly won another vote over her Brexit customs bill - but only after reluctantly accepting amendments put forward by Brexit hardliners.
The government won by just three votes, underlining the fragility of May's support as she tries to find a way to move the complex Brexit process forward.
May's challenges came as some British politicians on Tuesday again questioned the legitimacy of the Brexit vote after the electoral watchdog said the official group campaigning for Britain's exit in the 2016 referendum broke electoral laws.
The Electoral Commission said the "Vote Leave" group, backed by senior politicians including former foreign secretary Boris Johnson, failed to declare 675,000 pounds (USD 894,000) it spent with Canadian data firm Aggregate IQ. The undeclared spending meant the campaign group exceeded the 7 million-pound legal spending limit by almost 500,000 pounds.
The commission said it found significant evidence that Vote Leave did this by funneling cash to a small, unregistered pro-Brexit youth group, BeLeave. Those who wanted Britain to leave the EU narrowly won the 2016 referendum, with 52 per cent of voters supporting Brexit.