The government has ramped up its rhetoric ahead of next week's public sector strikes, saying the action could hurt jobs and cost the economy half a billion pounds.

In a briefing session with journalists, Danny Alexander said there would be no more concessions from the government as there was "no money on the table." The chief secretary to the Treasury said he may bring his child to work with him as strikes are expected to close schools.

But TUC head Brendan Barber said the figures were "fantasy economics" and ministers were using the strike as a "desperate excuse" for low growth.

“Dedicated public sector workers take no pleasure in taking action next week but the blame for this strike lies squarely with the government for failing to engage in serious talks until unions decided on a day of action", he said in a statement on Thursday.

The comments came after the news that the Home Office has contacted government employees to ask if they will cover ports and borders while the UK Border Agency workers strike over pensions.

And staff working abroad in immigration and in embassies have been contacted about strike cover.

The agency faced pressure recently over relaxed borders. The row led to the suspension of UK Border Force official Brodie Clark, who maintains he did not overstretch his remit when relaxing checks on some passengers entering Britain.

The most senior official at the Home Office, Dame Helen Ghosh, has fully backed Theresa May's account of the UK Border Fiasco.

PCS general secretary Mark Serwotka said the plea for civil servants to work on borders was "an insult". "It is an insult to hard-working border force staff that the government is scratching round trying to fill skilled posts with people who are essentially volunteers."

A UK Border Agency spokesman said: "The security of the UK border remains our top priority and we explore all options to ensure we minimise any disruption caused by planned union action."