The UK’s opposition Labour Party effectively extinguished all prospect of a second referendum on Britain’s European Union membership as Prime Minister Theresa May headed for Wales, where polling suggests she will win most of the seats for the first time in more than 150 years.

Keir Starmer, Labour’s Brexit spokesman, offered to soften the Brexit blow by immediately guaranteeing the rights of EU citizens to stay in the country if Labour wins the June 8 snap election. In Wales, where May stands a good chance of ending Labour’s dominance, the premier will make the case that a vote for her will strengthen her hand in negotiations.

In an election haunted by the ghost of Brexit, Labour’s latest outline will come as a blow to the 48 percent who voted to remain in the bloc and are unsure whether their vote can even matter given May’s crushing advantage in the polls.

Starmer said the party ‘‘genuinely accepts the outcome of the referendum’’ — a concession that may drive those who want to reverse course into the arms of the more pro-EU Liberal Democrats.

Bloomberg News