Train passengers face a summer of chaos after the Government signed off mass cancellations as a result of the the "pingdemic".

At least seven major rail networks are planning to impose emergency timetables for up to six weeks due to a critical shortage of train drivers and crew. One industry source said: "Cancellations galore are coming down the line."

On Friday, Downing Street moved to tackle the disruption by including some train drivers in a new mass exemptions scheme, following similar moves for the food production industry to keep food on the shelves.

About 100 sites in the transport and freight industry will be allowed to deploy daily Covid testing instead of self-isolation for a limited number of workers. A further 100 will be set up for frontline police and fire services as well as Border Force staff.

However, the Government’s plans were thrown into doubt after Britain’s biggest union encouraged its members to self-isolate even if exempt.

Christina McAnea, the general secretary of Unison, which has 1.4 million members mainly working in the public sector, told The Telegraph: "Anyone pinged or called by Test and Trace should isolate. Staying at home protects colleagues, neighbours, friends and everyone else too.

"No one should be forced into work when isolating, even those employees on the exempt list."

Are you exempt?

Roughly five per cent of train drivers – an estimated 1,250 people – are now understood to be absent, with the majority having been pinged by the NHS Covid app. London Northwestern Railway said the number of staff having to quarantine had "quadrupled".

Dozens of routes were cut on Friday due to staff shortages, while passengers on one train to Exeter were left stranded in Salisbury after being kicked off in the middle of their journey.

Officials at the Department for Transport are understood to have signed off plans to run the reduced services as families head off on summer "staycations". Industry insiders said that it was necessary to "make the most of the reduced resources".

ONe source said: "Without this, you would have the chaos of last-minute cancellations."

George Eustice, the Environment Secretary, admitted passengers should expect more trains to be cancelled "as a consequence of staff absence", telling the BBC: "We recognise that’s a reality over these next few weeks."

The new timetables will run for between four and six weeks on Northern, Avanti West Coast, Great Western Railway, West Midlands, Govia Thameslink and LNER.

Southern will cut its weekday timetables on five routes from Monday "until further notice", while South Western Railway said it had "made pre-emptive changes to some services this weekend".

Transport for London said London Underground’s Circle and Hammersmith and City lines would be closed this weekend due to more than 300 staff self-isolating. There will also be changes to services on the District and Metropolitan lines.

One former Tory transport minister called on the Government to immediately let all train staff conduct daily tests rather than self-isolate as staff shortages lead to cancellations.

"None of this makes any sense. Either we’re confident in the vaccines or we’re not," the former minister told The Telegraph. "Having our public transport system grind to a halt because of an app that appears to behave strangely at the best of times is utterly daft."

A government source working on transport policy stressed that capacity on train lines was only about 45 per cent of normal levels because of the pandemic, minimising the impact of disruptions.

A Rail Delivery Group spokesman said: "Like other sectors, some of our colleagues are having to self-isolate after being pinged and this means trains have to be cancelled, sometimes at short notice. 

"The Government’a exemption scheme may help to reduce extreme disruption, but it won’t stop cancellations being necessary on some routes so we advise people to continue to check before they travel or sign up for alerts about their journey from National Rail or their train company."