Ineos Britannia’s bid to win the 37th America’s Cup received a huge boost on Monday after they won a dramatic sail-off against Italians Luna Rossa to finish top of the round-robin stage in the Louis Vuitton challenger selection series.

It means Ben Ainslie’s team will get to choose their opponents for the semi-finals, which begin in Barcelona on Saturday. The winner of the Louis Vuitton Cup gets to race against New Zealand in next month’s America’s Cup match.

Monday’s sail-off took place only because Luna Rossa were dramatically disqualified from their final round-robin match against Swiss challenger Alinghi after developing an issue with their board arm in the pre-start, forcing them to veer off course.

The automatic disqualification left the Italians level on six wins apiece with GBR, who had earlier beaten France in their final round-robin match, a victory which eliminated the French from the series.

GBR did not pass up what Ainslie later admitted was an “unexpected opportunity”.

Luna Rossa may have been the form team for virtually the entire round-robin phase of this series, opening their account with six wins from their first six races against their fellow challengers. But Ineos - whose AC75 has been developed with partners Mercedes F1 - handed them their first defeat on Saturday, an impressive win in a reasonably heavy breeze. And they followed that up with an equally commanding victory in lighter winds on Monday, suggesting they can be competitive across the wind range.

Coming in on the port tack in the sail-off, Ainslie and co-helm Dylan Fletcher did a better job than Luna Rossa’s Francesco Bruni and Jimmy Spithill in the pre-start, getting their time and distance spot-on and hitting the line with more speed than their rivals.

From there, Britain controlled the race beautifully, tacking in front of the Italians wherever possible on the first upwind leg to give them their dirty air. The British challenger of record reached the top gate with an 18-second advantage and never looked back. By the finish, that gap was up to 42 seconds.

Ainslie’s team are not obliged to reveal who they want as their opponents until Friday’s pre-semi-finals press conference. Their choice is between Luna Rossa, American Magic - who have had late drama of their own with British co-helm Paul Goodison having to quit the series for now because of fractured ribs - and Alinghi.

The likelihood is they will go with Alinghi. The Swiss challenger had a slow start to the series and might have been sucked into their own sail-off for bottom spot had France beaten Britain and Luna Rossa not developed a technical issue. They have looked stronger in recent days, though, and Ainslie was certainly not giving anything away afterwards.

“To win the America’s Cup, you’ve got to beat the best,” he said on ACTV when he was asked who he was going to pick. “To be honest, we weren’t really expecting that [finishing top of the round robins]. It is a real credit to the team, both here in Spain and back home in the UK at Mercedes.”

Asked about the boat’s improvement through the series, Ainslie added: “We’ve said all along this is a development game. OK, we started off a little bit rusty in [last month’s] preliminary regatta. But every day we’re getting better with the sailing, with the technique. We’ve just got to keep it going.”

Fletcher, who was a late call-up in place of Giles Scott, was modest when asked to take some of the credit for Britain’s strong form in the starts. “Obviously I’m quite inexperienced, so Ben’s doing a great job of teaching me. Also our start coach Ian Williams and Giles Scott.”

Spithill, who won the Cup for the United States in 2010 and 2013 and now races for Luna Rossa, may have had his tongue in his cheek when he told ACTV he was “already looking forward to getting stuck into these semis and sailing against the Americans”. But it was probably an accurate prediction.