Alliance Trust Savings has paused new business on the platform as it begins to move assets over to a different technology provider.
A spokeswoman from the platform has confirmed that new business was halted on 1 December in order to prepare for the migration of legacy business onto the new technology provider early next year.
The move will see £12bn assets under administration move over to GBST Composer from its current technology provider Activebank.
Despite pausing new business on GBST, Alliance Trust Savings is still accepting new business through its current provider.
Last month, AJ Bell's chief executive Andy Bell pointed out that more than half of the assets in the UK platform market are on the verge of migrating over to different IT systems.
He described this as "scary", pointing to the hefty costs incurred from re-platforming.
The spokeswoman from Alliance Trust Savings said the platform had been working closely with advisers to move existing advised clients over to GBST.
“In order to prepare for the active migration, we are pausing further new business onto Composer until that time so that we can ensure a smooth transition," she said.
“We will continue to use Activebank for new business in the interim.”
The spokeswoman said business would reopen again in the first quarter of 2017.
Dan Farrow, director of SBN Wealth Management, said the decision by Alliance Trust to pause the platform was "sensible".
"It appears that Alliance Trust are rightly running a parallel system and have decided to sort out any issues they have with the new offering before transferring more assets.
"We are still using them for new business cases on the old system and as far as we’re concerned, it’s business as usual.”
In a joint statement between GBST and Alliance Trust Savings, a spokesman said: “There is no issue with GBST Composer or the ATS advised platform.
He said there had been a pilot and “the few firms” involved have been asked to pause placing new business.
He said: “It’s a commonplace and sensible phased approach that we know many advisers appreciate.”
He added: "This very brief pause is simply to ensure a smooth transition from ActiveBank to the new Composer technology.
“Advisers can still place new business with ATS without any disruption to service.”
This article was updated with comment from GBST and Alliance Trust Savings on 8 December.