* Dispute over conditions/work practices
* Union says national security at risk
LONDON, Oct 12 (Reuters) - About 2,500 staff working for Britain's border authorities will begin a two-day strike on Wednesday in a dispute over conditions and working practices, causing disruption for travellers, their union said on Tuesday.
It said the walkout would cause serious problems for travellers at all of Britain's ports and airports, and would include staff working at border crossing points in Calais, Dunkirk and Coquelles in France.
Britain's interior ministry said it would try to get passengers through passport controls as quickly as possible during the strike.
The Public and Commercial Services Union (PCS) said proposed changes to shifts and roles of passport control staff working in the Border Force, part of the UK Border Agency (UKBA), would increase costs and could put national security at risk.
The action comes as British unions warn of an increase in industrial unrest as deep spending cuts planned by the Conservative-Liberal Democrat coalition to curb a record peacetime budget deficit bite.
Next week, the government is due to outline cuts of 25 percent in public spending over the next four years.
The PCS said UKBA wanted to alter the current flexible rotas which allocated staff to passport controls on a daily basis and provided round the clock coverage, 365 days a year.
It also argued that a proposed downgrading of EU passport control would limit the ability to detect forgeries and national security could be at risk if the role was performed by less experienced staff.
"Far from saving money, these plans will increase costs unless there are also job cuts, and we believe this is what UKBA is planning," said PCS general secretary Mark Serwotka.
"Our members perform vital roles in keeping our borders secure and we cannot accept cuts or changes that put this at risk."
Lin Homer, UKBA chief executive, said the strike was over efforts to modernise working practices to provide a more efficient service.
"We expect the vast majority of our facilities and services to remain open for business," she said in a statement. "We will work hard to keep any possible delays to a minimum."
(Reporting by Michael Holden)