The company last week controversially announced that the daily service – from Shrewsbury to London – had been cancelled for the foreseeable future.
The move sparked outrage from local politicians, who said they had not been notified ahead of the change, and said they would be pressing the issue with the government.
But, Avanti now said the service will be running – from Monday to Friday – apart from days the company is affected by strike action.
The company says weekend running of the service is however still not definite and will be subject to confirmation.
A spokesman said: "We are pleased to confirm that our direct service from Shropshire to London will continue to run for the duration of our reduced timetable.
"On Monday to Friday the service to London will depart Shrewsbury at 6.39am, returning from London Euston at 6.23pm."
The cancellation had been part of a reduced timetable the company brought in as a reaction to what it said was a combination of issues – staff shortages due to strike action, sickness, and "the majority of drivers making themselves unavailable for overtime in a co-ordinated fashion, and at short notice".
The cancellation had been criticised by Telford & Wrekin Council leader Shaun Davies, Conservative MP for the Wrekin, Mark Pritchard, and his party colleague Shrewsbury & Atcham MP Daniel Kawczynski.
The situation means that Shropshire is however still short of a second daily direct service, with the second cancelled at the start of the pandemic yet to be re-instated.
Councillor Shaun Davies said: “Avanti clearly made the wrong decision to suspend this service last week, putting additional pressure on Transport for Wales (TFW) and West Midlands Trains (WMT). I made my position very clear, as did others, that this was totally unacceptable and a massive blow to Telford and Shropshire.
“Whilst I am very pleased that Avanti have clearly listened to our concerns and have reinstated the service, I’m disappointed they made a poor decision in the first place without notice or indeed consultation. If this had not been challenged our residents would have been left with no direct service to London.
“We are on the side of local people and will continue to confront anything that disadvantages them.”
Councillor Richard Overton, deputy leader and cabinet member for transport, added: “It’s very good news that Avanti have now found the resources to reinstate the direct service between Wellington and London. It was the wrong decision, undermining confidence in rail travel, and it must not happen again.
“Indeed we need to improve the service further and we are calling for the second direct train to London, that stopped running before the pandemic, to be put back into action. We know that a direct rail service to the capital is important for so many people, for those who need it for work and also for day trippers.
“Additionally we will continue to campaign for a better service to Birmingham, as well as the urgent electrification of the line, so that more people can use efficient and effective trains and benefit from improved public transport links to the city.”
Mr Pritchard said: "If true this must be one of the quickest, and most welcome U-turns in recent railway history.
"In reality the timetable needs to be expanded and not scrapped. I welcome this change of heart but Shropshire, and Wellington passengers in particular, need certainty, not confusion and more uncertainty.
"Maybe Avanti made the big mistake of thinking MPs don't pay attention in recess. Well clearly some MPs do."
Mr Kawczynski said: "I met with the Rail Minister Wendy Morton this morning to reiterate to her that it is essential the service is back up and running as soon as possible and she needs to hold Avanti to account.
"The current strike situation must not be allowed to impede the guarantees we are seeking from this company and its obligation to provide the service."
RMT strikes are planned on Thursday and Saturday when all services in Shropshire are again expected to be cancelled.