Around 6,000 small and medium sized businesses employing around 1 million people will come under the government’s auto-enrolment pension rules tomorrow.

Auto-enrolment means firms must place qualifying members of staff on a company pension automatically.

Businesses must contribute to the pension, as well as keeping track of membership and make records of the arrangement with each worker.

The rules have been introduced gradually since 2012, with the biggest firms being inducted first, followed by gradually smaller organisations.

Accountancy firm Irwin Mitchell says many businesses have struggled with the transition.

It says 33 per cent of firms have taken on new staff to cope with the system.

Meanwhile, it says a third of firms already running auto-enrolment said they wished they had given themselves more time to prepare.

Tomorrow, organisations employing between 160 and 249 staff will need to be ready. More firms will gradually stage over the next four months.

But only 28 per cent of smaller businesses said they would struggle with the administrative burden.

Nigel Bolton, pensions partner at Irwin Mitchell, said: "Larger businesses that have already staged find the process a lot more challenging than they expected; smaller firms are underestimating what the true impact of auto-enrolment will be.”