The UK can "turn the tide" on the coronavirus crisis within 12 weeks, Prime Minister Boris Johnson has said. But pressed on what he meant by the three-month timescale, he said he did not know how long it would go on for. He said trials on a vaccine were expected to begin within a month and warned he would "enforce" Londoners to be kept apart "if necessary". Earlier, in a message to the nation, the Queen urged people to come together for the common good. Speaking in Downing Street, Mr Johnson told reporters: "I believe that a combination of the measures that we're asking the public to take and better testing, scientific progress, will enable us to get on top of it within the next 12 weeks and turn the tide. "I cannot stand here and tell you that by the end of June that we will be on the downward slope. "It's possible but I simply can't say that that's for certain," he added. "We don't know how long this thing will go on for. But what I can say is that this is going to be finite." Earlier Sir Patrick Vallance, the UK's chief scientific officer, was also asked how long it would go on for. "We are dealing with a brand new virus. We are dealing with learning as we go on and I think now to put absolute timelines on things is not possible." In the press conference on Thursday evening, Mr Johnson ruled out closing down public transport in London but pointed out people in some parts of London were not following government guidance on social distancing and would be "enforced" to do so if necessary. "I know how difficult it may be or it may seem right now, but if we do this together, we will save many, many thousands of lives," he added. He also urged businesses to stand by their employees "because we will stand by you", adding that his chancellor would be making further announcements on Friday. Later, Health Secretary Matt Hancock pledged to rush protective personal equipment (PPE) to frontline NHS staff and social care providers, following concern workers were being put at risk by shortages. Speaking on BBC1's Question Time, Mr Hancock said the UK had shipped 2.6 million masks and 10,000 bottles of hand sanitiser in the last 24 hours. He promised that "overnight 150 hospitals will get the next pack of protective equipment they need… every single hospital will get their next batch of equipment before Sunday night". He also confirmed social care providers would get a package of personal protective equipment by the end of next week. In response to a government appeal for more ventilator makers, 1,400 companies had offered to switch their operations to help manufacturer them, including Formula One, he said. He added that officials would work with leading companies to radically increase the number of coronavirus tests, after the government pledged 25,000 tests per day within four weeks. He also said the government has bought a test which can detect whether someone has had coronavirus - and their immunity to it. The press conference came as Italy's death toll rose by 427 to 3,405, overtaking China's toll. In the UK, 144 people with coronavirus have died, and 3,269 people have tested positive for it. Speaking alongside the PM, Prof Chris Whitty, the UK's chief medical adviser, warned there would be a "lag" before the public's efforts to stem the spread of the virus would slow down case numbers. He said there would be a "global issue" in the supply of personal protective equipment (PPE), saying NHS workers were right to complain about the shortages.