Andy Burnham’s likely chief of staff will move to Downing Street from a lobbying firm that counts Thames Water and the rival bidder for Heathrow’s third runway as clients, just as ministers mull decisions that could decide both organisations’ fate.

James Purnell is widely tipped to become Burnham’s most senior right-hand man should the Makerfield MP succeed Keir Starmer as Prime Minister as is expected, rekindling a professional relationship that dates back to when the pair served in the same cabinet under Tony Blair.

Purnell, a former work and pensions secretary, has forged a successful career in the private sector after quitting politics in 2010, most recently running Flint Global, a London-headquartered advisory that counts blue-chip British and global firms among its roster of clients. He stood down from the role on Wendesday morning, telling staff on Tuesday he was “likely to be leaving shortly“.

The firm specialises in advising companies on complex policy and reputational issues, often lobbying lawmakers and regulators on their behalf. Unlike most UK-headquartered lobbying firms, Flint does not disclose its client base publicly.

According to people familiar with the firm’s work, however, it has spent several years representing Heathrow Reimagined – a campaign to overhaul Heathrow’s regulatory model – and its ringleader Surinder Arora, the man behind the rival third runway expansion. It has also worked closely with the embattled utility Thames Water and wider regulated water industry.

Thames Water among Flint’s client base

The pair of Flint clients are both expecting imminent decisions from central government that will decide their fate.

Thames Water is currently teetering on the brink of bankruptcy and faces the prospect of being brought into public ownership after the government objected to a £10bn rescue proposal put forward by the company’s bondholders. Environment secretary Emma Reynolds said the proposal did not go “far enough to protect customers and the environment”.

Flint is understood to have helped advise on the terms of the package, and has long been a representative of “water companies and regulated institutional investors” in the industry. According to the firm’s website, it has helped several water firms “navigate price control processes” and advised on “policy and political risks”, engaging regulators and lawmakers.

Burnham has previously suggested he is in favour of bringing the existentially threatened utility under public ownership. He told The Guardian this month that nationalisation of Thames “should be done”, having previously promoted the taxpayer having “greater control” of water companies.

Purnell’s Downing Street in-tray will also contain decisions on the future of Heathrow, the expansion of which has been one of the Starmer government’s flagship measures in its bid to revive the UK’s stuttering growth trajectory.

Flint has worked with hotel magnate Surinder Arora for several years, helping shape his airline-backed Heathrow Reimagined campaign to change the west London hub’s regulatory model. The lobbying firm has also played a role advising on Arora’s bid to oversee the totemic third runway expansion, which could see a project at the west London hub operated by a rival bidder for the first time in the airport’s history.

At a planning decision last week, transport secretary Heidi Alexander kept Arora’s hopes of running the megaproject alive, tacitly endorsing the billionaire hotel magnate’s plan for a phased approach to the expansion’s construction.

‘No ongoing financial interest in the company’

Purnell, a New Labour architect who served in Tony Blair’s policy unit before becoming a minister, announced his plans to step down from his role as Flint’s chief executive in a memo to staff on Wednesday. Writing to staff, he said: “I am likely to be leaving shortly,” adding that the company’s co-founders Ed Richards and Simon Fraser would take on the roles of interim chief executive and chair respectively.

In his letter, Purnell also said he “looked forward to keeping in touch” with Flint staff.

A spokeswoman for Flint said: “James has resigned from Flint. James has recused himself from all client activity and will have no ongoing financial interest in the company of any kind. James decision to join Burnham’s team is entirely his own and one we fully respect.”