Picket lines at the Royal Courts of Justice are said to be holding

Some 20,000 staff in four civil service departments, including court ushers, clerks and immigration officers, have joined the 48-hour walkout over pay.

The Public and Commercial Services Union said the picket lines were holding despite the cold weather.

But government departments denied their work was being seriously hampered.

The action by PCS members in the Home Office, the Department for Constitutional Affairs, the Prison Service and Treasury Solicitor's Department follows offers of below inflation pay rises.

'Endemic low pay'

These range from 0.5% for Treasury Solicitor's Department staff to 1.3% for court clerks and ushers in the Department for Constitutional Affairs.

PCS general secretary Mark Serwotka, who joined an early morning picket line outside the Royal Courts of Justice in London, said low pay was "endemic" in the civil service.

He accused management in the four government departments of trying to "drive down" pay, adding that many of those on strike had never taken industrial action before.

The union had been left with no other option but to strike, he added.

These are not your bowler-hatted Sir Humphrys PCS union

One out of every four civil servants earned just �13,750 a year and some of those on strike were earning less than �10,000, the PCS said.

"These are not your bowler-hatted Sir Humphrys," a PCS spokesman said.

"They are people dealing with asylum claims, helping asylum seekers and people who are ensuring that the justice system runs smoothly," he added.

The union said picket lines were holding outside immigration offices and civil and county courts where a skeleton service was being maintained.

The Manchester Immigration Appellate Authority had been closed by the action and managers had been shipped into immigration offices in Wales, the PCS said.

Restricted service

But the Home Office denied any disruption had been caused to its immigration offices by the action.

A spokesman for the Constitutional Affairs department said it hoped to keep all courts open but acknowledged there may be a restricted service in some areas.

The strike is set to continue throughout Friday unless there is a breakthrough.