The business process outsourcing group said Ellerton had stepped down for personal reasons.
DHL Express chief executive Ken Allen will run the £400m-plus turnover group while a replacement is sought.
A Williams Lea Tag spokeswoman told PrintWeek: “On an interim basis, Ken Allen, CEO DHL Express and board member of Deutsche Post DHL Group, Williams Lea Tag’s parent company, will directly lead the Williams Lea Tag business (in addition to DHL Express), until we find a successor.”
Allen is a chartered accountant who has worked at DHL Express since 1985, most recently in a variety of senior roles at its worldwide operations. He will retain his responsibilities for DHL Express during this period.
There is also industry speculation that Deutsche Post DHL could be looking to sell Williams Lea Tag.
“They are due to appoint advisers in the next few weeks,” said a source close to the company.
A Williams Lea Tag spokeswoman said the firm did not comment on market speculation.
Williams Lea Tag was dealt a blow earlier this year when the Cabinet Office rejigged its £300m Crown Commercial Services framework, which had previously involved a large single-supplier contract held by Williams Lea Tag.
It is now one of seven suppliers for the new framework, with a transition period due to end in June.
Deutsche Post acquired a majority stake in Williams Lea in 2006 in a deal that valued the business at £450m. Williams Lea went on to buy The Stationery Office later that year for £130m, followed by pre-media and production agency Tag Worldwide in 2011.
The group rebranded the Williams Lea operation as Williams Lea Tag last year. It employs around 11,000 people worldwide.
Williams Lea had flat sales of £362m in the year to 31 December 2015, but operating profits fell by 32% to £11.9m and the business had £5.4m in restructuring costs “in relation to ongoing operating model changes that will better deliver integrated marketing and supply chain solutions”.
Sales at The Stationery Office in the same period were down 3.6% to £46.2m, while operating profits fell by 20% to just under £8m, in part due to the period of ‘purdah’ required prior to a general election.