TSB's ill-fated computer upgrade was beset by huge glitches in the months leading up to its launch, a whistleblower has claimed.

Many customers are still locked out of their bank accounts more than two weeks after TSB's failed system switchover from its old owner, Lloyds Banking Group, to Spanish company Sabadell.

A former employee told Money Mail that senior staff had played down warnings from IT workers.

Ongoing: Many customers are still locked out of their bank accounts more than two weeks after TSB's failed system switchover

The man, who worked on the online banking migration team, says the chaos following its launch came as no surprise to him.

Just weeks before the deadline, staff were frantically removing non-essential services because so many things still weren't up and running, he says.

He alleges that staff raised concerns about the new system every week during a two-hour steering meeting. These were then compiled into a 30-page report and passed to senior staff.

The most serious concerns were highlighted red, lesser worries amber, and defects that had been resolved green.

The former TSB worker claims that, as the changeover date approached, staff felt under pressure to 'massage' these alerts and change red issues to amber or even green, despite the problems not being fixed.

'It was widely known there were a lot of issues. But the attitude was: 'We'll be OK',' he says.

He alleges that many of the bugs being reported now — such as error messages written in IT code that customers are seeing when they try to log in — had been flagged up months ago.

He believes the team was understaffed and many lacked expertise.

He also alleges the language barrier between TSB staff and those at Sabadell meant they were never sure whether the defects they had reported to the development team in Europe were being fixed.

A TSB spokesperson says: 'We do not comment on speculation. However, extensive testing was completed before migration and we will be conducting our own independent review into why the issues that customers have been experiencing have occurred.'

v.bischoff@dailymail.co.uk