Legacy Richards Butler salaried partners are to have 20% of their annual pay held back at the start of next year, as part of a package of measures designed to tie them into the firm and bill more quickly.

Under the controversial scheme, the salaried partners will be paid the final portion of their remuneration at the end of the year, with the amount determined by the firm's performance.

The system was originally introduced at the beginning of the year following Reed Smith's merger with Richards Butler, when the proportion of pay being retained was set at 15%.

The policy brings remuneration at the US firm's European operations into line with its own pay structure.

In a further measure, designed to improve billing at the firm, all partners who fail to meet billing targets are having an additional percentage of their salary deferred on an ongoing basis.

Salaried partners can have 1.25% of their pay held back each quarter, to be repaid when they bring in the money owed to the firm. Equity partners who fail to bill on time face having up to 18% of their remuneration held back.

European corporate partner Michael Pollack said: "Since we introduced this system we have improved our performance in terms of getting bills collected."

A senior partner at a rival firm said: "It is obviously a way of catching out salaried partners who are not pulling their weight."

One legacy Richards Butler partner said: "My view is that this is sensible and it works."

However, the move has been criticised by other partners at the firm, with one commenting: "Your budgeted salary becomes conditional on things out of your control – the resentment among some partners is astonishing."

The system also means that partners leaving before the end of the financial year risk forfeiting their outstanding salary, although the firm is thought to be cutting deals with departing partners.

It is understood that when former Reed Smith business immigration partner Caron Pope joined CMS Cameron McKenna, Camerons agreed to underwrite certain billing targets so that she could leave before the end of a quarter.

The news comes as a survey by investment bank Citi this week found salaried partners to be the least productive tier of lawyers in the US.