Prime minister Boris Johnson led tributes to Gareth Southgate’s triumphant England team on Thursday as the country celebrated reaching its first major men’s football final in 55 years.

Following a tense 2-1 semi-final victory over Denmark in front of 66,000 fans at Wembley Stadium — and many millions more watching on television — England’s players banished decades of underachievement to book a place in Sunday’s showdown with Italy.

“It was a total nail-biter,” said Johnson, who hasn’t ruled out a public holiday if England go on to win the European Championships. “It was just euphoric, absolutely extraordinary. I congratulate Gareth Southgate and the whole team, I thought they were stunning.”

Despite the scenes of jubilation that greeted the result, England still required extra-time and a fortunate rebound from a penalty kick initially missed by star striker Harry Kane to defeat a resolute Danish side that was kept in the game by goalkeeper Kasper Schmeichel.

The result was also slightly marred by accusations of questionable tactics to gain an edge, as well as a disciplinary matter.

Raheem Sterling, the forward who won England’s penalty, was accused by Danish and Italian media of falling theatrically under minimal contact to earn the spot kick.

The referee’s decision stood after being reviewed by the video assistant but José Mourinho, the former manager of English Premier League side Tottenham Hotspur and new coach at Italian team Roma, claimed it was “never a penalty” even though the “best team won”.

Italy’s midfielder Marco Verratti diplomatically described it as “a bit of a generous penalty” but predicted an “epic final”.

England fans are also facing scrutiny from Uefa, European football’s governing body, which took disciplinary action after a laser was shone in Schmeichel’s face in the build-up to the decisive spot-kick. Uefa also launched an investigation against England over a “disturbance” during Denmark’s national anthem and fireworks being set off.

Sections of the England support had already been criticised for booing opposition national anthems and their own players’ prematch anti-racism gesture of “taking the knee”.

Meanwhile scientists warned the Euros could be behind surging infections, particularly in men, as people gather in stadiums, living rooms, pubs and on the streets to watch football and celebrate England’s success.

Pictures of central London after the match showed huge crowds of fans, with some climbing on the top of buses, gathering to celebrate with little evidence of social distancing or the wearing of face masks.

The Metropolitan Police in the capital said on Twitter that officers had arrested 20 people for various offences, including common assault, with “large pockets of groups” gathering to mark the victory.

For manager Southgate, who has become the first England boss since 1966 World Cup winner Sir Alf Ramsey to take the national team to the final of a major international men’s tournament, the support was a decisive factor.

“The fans were incredible all night,” he said. “I’ve not heard the new Wembley like that — ever.”

But there are fears such widespread ebullience is contributing to the increase in coronavirus cases ahead of July 19, when the government plans to lift almost all restrictions introduced in England to limit the spread of the virus.

In a letter to The Lancet, a medical journal, a group of scientists and health experts warned that the government’s plans to open up are a “dangerous and unethical experiment”.

Separate research published on Thursday showed that the reproduction number for Covid-19 in England jumped to almost 1.9 in early July, which means that every infected person transmits the virus to a further 1.9 other people. Imperial College London’s React-1 study, which looked at almost 50,000 swab tests taken between June 24 and July 5, found that men were 30 per cent more at risk.

“It could be that watching football is resulting in men having more social activity than usual,” said Steven Riley, professor of infectious disease dynamics at Imperial.

The Euros were increasing the probability that people were meeting indoors more frequently, he said.

A peak of 27.6m viewers watched England beat Denmark on ITV, STV and online via the ITV Hub, marginally ahead of the 27.5m audience that tuned in to England’s loss against Croatia in the 2018 World Cup semi-final.

Kate Nicholls, chief executive at trade body UKHospitality, said she welcomed the government’s decision to extend opening times for pubs on Sunday night so “fans can watch — and hopefully celebrate — the [final] in safe, supervised environments”.

The government and the London mayor Sadiq Khan are considering options for “safe” celebrations in the event that England beat Italy.

However, to win England must overcome an Italian side that is undefeated in 33 matches and fresh from the euphoria of beating Spain in a penalty shootout in the tournament’s other semi-final on Tuesday night and the much fancied Belgians in the quarter-finals.

“They are, as always, difficult to score goals against, and without doubt deserve to be in the final, having beaten two top teams to get there in Belgium and Spain,” said Southgate. “So it’s the biggest possible test we can have.”