Eurostar rail workers from two unions are to take part in two concurrent periods of strike action this month in a dispute over their work-life balance.
RMT members will walk out from 12 to 15 and 27 to 29 August while TSSA members will strike on 14 and 15 and 28 and 29.
Transport Secretary Chris Grayling said he was "very disappointed the unions keep calling strike action over what appear to be pretty minor matters".
Eurostar said all passengers with reservations would be able to travel.
Eurostar runs services between London and mainland Europe and the walkout coincides with the Assumption Day bank holiday in France and Belgium on 15 August and the 29 August bank holiday in England, Wales and Northern Ireland.
A spokesman said: "On the days of the strike we have made some small modifications to our timetable to ensure that all passengers booked to travel will be able to travel on those days.
"Customers affected by any changes will be notified in advance with options for their journey."
The company added that it had been planning for possible strike action and was still in discussions with the RMT over the dispute, which the union says concerns unsocial hours and duty rosters for about 80 train managers.
'Quality time off'
The RMT says Eurostar has failed to honour an agreement from 2008 over conditions for their staff.
General secretary Mick Cash said: "Our train manager members at Eurostar have a heavy commitment to shift work and unsocial hours and are sick and tired of the company's failure to honour agreements.
"Our members have every right to have a fair work-life balance that fulfils the operational needs of the company while guaranteeing quality time off for friends and family.
"It's now time for Eurostar to come to the negotiating table with a set of proposals that honours our agreements and guarantees our members a genuine work-life balance."
Manuel Cortes, general secretary of the TSSA, said: "This is a long-standing issue and there is still time to resolve it if common sense prevails...
"No-one wants to see disruption at holiday times and we will be working for a peaceful solution."
'Turn the clock back'
The announcement comes amid current and planned strikes by RMT members on Southern and Virgin Trains East Coast.
Southern Rail staff are in the midst a five-day strike over the role of conductors, while the union announced on Tuesday that its members on the Virgin East Coast line had voted to walk out in a dispute over staffing changes.
The RMT says the action involving Southern Rail will be suspended on Thursday and Friday if the company agrees to urgent talks without pre-conditions.
Speaking after the Eurostar strike announcement, Mr Grayling said: "This feels like an excuse to be militant and it's an extraordinary contrast.
"We're announcing a massive programme of modernisation of the railways, improving the situation for passengers.
"At the same time we have the unions trying to turn the clock back and hang on to working practices that are decades out of date. Passengers should not be made to suffer like this."