* Lawmakers publish IBM report into TSB's IT failure
* Report highlights insufficient testing
* TSB says report's conclusions are preliminary (Adds later TSB statement)
LONDON, June 21 (Reuters) - Britain's TSB Bank failed to sufficiently test a new IT platform before a botched roll-out that left thousands of customers locked out of their accounts, a report commissioned by the lender showed on Thursday.
TSB's management, and in particular its CEO Paul Pester, have come under fire from lawmakers over the crisis, which lasted for weeks, with some customers unable to make vital payments.
The International Business Machines Corp report was produced after TSB called in IBM to help fix the problem, which arose when the bank tried to migrate customers onto a new IT platform in April. The report was published by Britain's Treasury Committee of lawmakers on Thursday.
IBM's "early findings" concluded that testing was relatively quick, did not show sufficient evidence of the system's capacity and did not apply the criteria needed to prove it was ready.
IBM said that in such a risky operation, it would expect to see "world class design rigour" and "test discipline".
"In a similar situation when IBM partnered with a financial organisation to migrate to a new core banking platform ... multiple trial migrations were conducted, rolled back and then remediated prior to launch."
A TSB spokeswoman cautioned that IBM's preliminary observations were made after just three days at the bank.
"To present this document as a clear view on what went wrong wouldn't be a fair reflection," she said.
In another statement issued later on Thursday, the spokeswoman added that the observations were made eight weeks ago and are therefore "very much out of date".
"The hypotheses were not final, nor were they a validated view of what went wrong or of the actions that have subsequently been taken. Without this context, this document could be misinterpreted to the detriment of TSB's customers."
Earlier in June, in an unprecedented move, the committee called on TSB's board to consider whether Pester should stay in his job.
The committee has asked to be provided with a copy of a separate report into the IT outage currently being produced by law firm Slaughter & May. (Reporting by Emma Rumney; Editing by Elaine Hardcastle and Adrian Croft)