People needing Covid-19 tests have been sent to a centre in an area under local lockdown, despite it being closed.
Rhondda Labour MP Chris Bryant said the situation in Clydach Vale, near Tonypandy, Rhondda Cynon Taf (RCT), was an "utter farce."
The site is run by Serco and slots are booked via the UK government-run online system.
The UK government, which oversees most of Wales' coronavirus tests, said testing capacity increased daily.
All those who booked at the Clydach site were contacted and advised to go to an alternative site in Abercynon where they will be tested, a spokesman said.
Richard Case, 45, from Llanharan booked a test at Clydach Vale at 10:00 BST on Thursday for his 13-year-old son Dylan, who has a sore throat and dry cough.
When he and Dylan arrived they joined a queue of "around 20 vehicles" but could see in the distance that people were turning around.
"I thought they were there having the tests and then turning around, but only when we got to the front of the line did we see that everything there, all the council offices up there, were completely shut off.
"And it wasn't until I looked on Twitter and saw Chris Bryant's tweets when I got home that I realised that the test centre had been shut."
Mr Case then logged back on to book another test. Again, the system offered him Clydach Vale, but he selected Abercynon instead where he got a test at 12:30.
The experience of being sent to Clydach Vale was "frustrating to say the least", he said.
"I can only imagine people that are trying to get there on public transport, who've got children, or whose symptoms are much worse - it's not very good."
RCT's Labour council leader Andrew Morgan told BBC Wales a mobile testing unit was originally set up at Porth and then moved to Clydach three weeks ago.
It was closed on Wednesday as demand had fallen but bookings were still being taken on Thursday, he said.
"It seems they didn't take it off the booking system so hundreds have been turning up there - many from outside the area," he said.
The Cwm Taf Morgannwg health board tweeted to say its teams were "working on this as a matter of urgency".
'Badly let down'
Mr Bryant said: "I'm grateful to the health board and RCT who are trying to sort this out - and I've contacted [UK government Health Secretary] Matt Hancock as well.
"But we feel really badly let down in the Rhondda at the moment."
Rhondda Cynon Taf was the second county to be put into local lockdown last month after a rise in coronavirus cases there.
A UK Department of Health and Social Care (DHSC) spokesman said: "NHS Test and Trace is providing tests at an unprecedented scale - 225,000 a day on average over the last week - with the vast majority of the public reporting no issues at all with the process.
"Testing capacity increases daily and we're on course to have capacity for 500,000 tests every day by the end of October - bringing in new labs that can process tens of thousands of tests a day, opening new test sites, and trialling new rapid tests that will give results on the spot."
A spokesman for Serco said the company "does not manage the booking system but we understand that all those booked have been sent a message by DHSC asking them to go to Abercynon where they will be tested. We have also sent one of our team to Clydach to redirect people to Abercynon."