A London Tube line has been suspended due to a staff shortage triggered by workers told to self-isolate by the NHS Covid-19 app.
The Metropolitan Line was not running on Saturday due to control room staff being alerted by the app in the morning.
Services on the Piccadilly and District line were also affected.
Richard Jones, London Underground’s head of network operations, said: “Due to a shortage of control room staff who are having to self-isolate following notification this morning via the Test and Trace app, there will be no service on the Metropolitan line for the rest of the day.
“This will also impact the Piccadilly line with no service between South Harrow and Uxbridge and no service on the District line between High Street Kensington and Edgware.
“Services are expected to resume on the Piccadilly and District lines at 9pm this evening.
“We apologise to customers for the disruption. London Underground tickets will be accepted on local bus services.”
The suspension comes after warnings to the Government over a “surge” in workers and medics being forced into self-isolation over coronavirus contacts when most restrictions end in England on Monday.
Transport unions have said there will be “dire consequences” next week when staff are “pinged” as the level of infections rise.
Rail, Maritime and Transport union (RMT) general secretary Mick Lynch previously warned that Monday “will see a surge in workers pinged with a self-isolation instruction next week”.
“Even at this late stage, the Government, the train operators and the bus companies should issue a clear, legally backed instruction that levels up the rest of the UK to the safety standards that will remain in force in Wales and Scotland,” he said.
Elsewhere in the country, passengers were warned of alterations on some Northern routes this weekend due to a number of front-line staff either testing positive for Covid-19 or having to self-isolate.
According to the National Rail Enquiries website, trains may be cancelled in both directions on services connecting Huddersfield, Sheffield and Lincoln, Sheffield with Leeds, Retford and York and between Leeds and Doncaster.
The Metropolitan Line suspension in London came as Wembley Stadium, near to Wembley Park station on the north-west section of the line, prepared to host thousands of fans for the rugby league Challenge Cup final at 3pm.
The station is also serviced by the Jubilee line which was still running, while Wembley Stadium and Wembley Central stations are also near the ground.