Ashurst has overhauled its pro bono scheme so hours count as utilised time, as a host of firms step up their effort to increase lawyer involvement in charitable activities ahead of next week's National Pro Bono Week.

The firm is introducing a target for all lawyers – including partners – to complete an average of 50 pro bono hours every year, with pro bono hours also set to count towards billable hour and bonus targets for the first time. The scheme is expected to be rolled out firmwide in the coming weeks.

Dispute resolution and pro bono partner Mark Clarke said: "We realised we were doing a lot of pro bono work but that it could have been more cohesive and people needed to receive greater recognition for it.

"Now we're trying to bring everything together with a new set of criteria. Under the new programme, pro bono hours will count as utilised time and therefore bonusable time, which is a significant change. At the moment it's discretionary, but from now on it will be utilisable as standard."

Elsewhere, Irwin Mitchell has set its associates and partners a minimum requirement of 30-50 hours corporate social responsibility (CSR) work per year in order to be considered for promotion.

Irwin Mitchell personal injury head and social responsibility chair Stuart Henderson said: "We have recently reviewed our pro bono strategy with a view to increasing our commitment against the background of cuts in state provision and ensuring our people view pro bono as part of the fabric of our business."

Among the top 20 UK law firms by revenue, Allen & Overy (A&O), Simmons & Simmons, Clyde & Co, Eversheds and Bird & Bird are among those that now count pro bono hours towards billable targets, with DLA Piper changing its policy on 1 May so that pro bono hours are credited in peformance reviews. The firm has also set its UK lawyers an annual target of 35 hours.

Of the magic circle, Clifford Chance posted the highest estimated value for lawyer hours spent on community and pure pro bono work during 2011-12, at £17m.

This value compares to a figure of £14.3m reported by A&O, while Linklaters valued its pro bono work at £3.5m. DLA Piper valued its global pro bono and community initiatives at $110m (£68.2m) in 2011, though at present there is no real industry standard for firms to use when estimating the value of such activity.

National Pro Bono Week kicks off on Monday (5 November), with many firms hosting events to promote charitable initiatives. CMS Cameron McKenna is staging its inaugural CSR awards, which will recognise significant employee contributions.

Notable initiatives launched by top law firms over the last year have included A&O's two-year tie-up with Ghana child rights organisation AfriKids.

It has allocated pro bono hours worth up to £1m to cover both fundraising activities for its global partner charity and the provision of pro bono legal advice.

Separately Linklaters is set to rebrand its corporate responsibility programme as 'collective responsibility' in an effort to highlight individual responsibility and demonstrate that it is "about 100% of what [the firm does] and not just the 1% volunteering or recycling".