Holidaymakers should not pin their hopes on a slew of extra countries being added to the quarantine-free green list when it is updated later this month, government sources have warned. With ministers monitoring data daily on the spread of the Delta variant, after stage four of the reopening roadmap was postponed by a month to 19 July, Whitehall insiders say the mood remains extremely cautious. Cases of the Delta variant are rising rapidly in the UK, with 11,007 new infections reported on Thursday – the highest figure since 19 February. Portugal was the only mainstream European holiday destination on the original green list, and it was then removed on 3 June as the Delta variant spread. The government has promised to update the list again by 28 June, with details likely to be announced next week to give travellers and holiday firms time to adjust their plans. “My sense is that we’ll continue to be very cautious in thinking about how we take any steps that could increase transmission,” said a government source. Another said the decision would be made by No 10, not the Department for Transport – and would be based purely on public health considerations. Boris Johnson has described the 19 July date as a “terminus” and is keen to avoid taking any steps that could risk a fresh delay. It came as Ireland’s chief medical officer advised the public against non-essential travel to the UK amid concerns over the Delta variant. Dr Tony Holohan said a decision on advice on travel to Northern Ireland from the Republic has not been formally made but said they are “concerned” about the situation across the border. Meanwhile, Downing Street has confirmed that ministers are examining the idea that double-vaccinated people could be allowed to return from amber list countries without quarantining – though any change is unlikely to take effect in the near future. The prime minister’s official spokesman insisted “no decisions have been made”. “We want people to be able to travel abroad as soon as it is safe to do so. Currently we have set out a traffic light system for international travel. We’re always learning more about the virus and its variants. At this stage in the pandemic, our current approach is the right one, but we keep our measures under review, and that was set out clearly in both the roadmap and the global travel taskforce report,” he said. The government’s four-step roadmap to reopening, published in February, said vaccinations “could offer a route” to the “safe and sustainable return” of foreign travel. “Once more is known about the evidence of vaccines on transmission and their efficacy against new variants, the government can look to introduce a system to allow vaccinated individuals to travel more freely internationally,” the roadmap said. The NHS app has been updated to include the user’s vaccine status, so that British travellers will be able to prove they are protected against the virus. Currently travel to countries on the amber list is strongly discouraged, aside from exceptional circumstances. Travellers returning from red list countries must quarantine in a hotel. Johnson has been repeatedly criticised by Labour for what they claim are his lax border policies. Keir Starmer argued at Wednesday’s prime minister’s questions that the rapid rise of the Delta variant in the UK could be blamed on Johnson’s failure to add India to the red list, from which travel is effectively banned, in early April. “The British people did their bit by following the rules and getting vaccinated but the prime minister squandered it by letting a new variant into the country. That was not inevitable. It was the consequences of his indecision,” he said. Johnson insisted the UK had the “toughest border measures anywhere in the world” – a claim hard to square with the fact that some countries, including Australia and New Zealand, bar almost all overseas arrivals.