Sky News understands that Powerscourt Group, which was founded by Rory Godson, a former journalist, in 2004, is on the brink of an agreement to be bought by Morrow Sodali, a US-based stakeholder advisory firm.

City sources said Morrow Sodali, which is itself backed by the private equity giant TPG, was expected to announce the deal this week.

One added that it could ultimately be worth as much as £50m.

Powerscourt has steadily climbed the rankings of UK financial PR firms in the two decades since it was founded, acting for clients such as Entain, the gambling group, Airbus and THG, the online health and beauty retailer.

It is currently handling mandates for the former BP chief executive Bernard Looney and the former NatWest Group boss Dame Alison Rose, both of whom stepped down in controversial circumstances this year.

In recent years, it has diversified to offer political advice, digital engagement and consulting on environmental, social and governance issues.

The firm's takeover will continue a trend which has seen sizeable deals involving several of its rivals over the last year.

Finsbury, founded by the former Financial Times journalist Roland Rudd, sold a minority stake to KKR, the private equity giant, earlier this year.

Tulchan, another independent firm set up by Andrew Grant, was sold to Teneo, a larger US-based firm offering services including restructuring and insolvency advice, for about £70m.

Mr Godson declined to comment.