Further strikes by airport and passport workers have been announced for next month as part of their escalating disputes over pay.

Security staff at Heathrow Airport will take a further eight days of industrial action in May, the Unite union said.

The Public and Commercial Services Union (PCS) has confirmed more than 1,000 Passport Office workers will also walkout at the start of May.

The FDA union will also ballot senior civil servants for strike action.

The latest announcements from the three unions come amid a wave of industrial action by hundreds of thousands of workers across different sectors over the past year.

Disputes are mostly over pay, with unions calling for wage rises to keep up with rising prices.

Almost 2,000 PCS members working as passport examiners in offices in Belfast, Durham, Glasgow, Liverpool, London, Newport, Peterborough and Southport have been on strike since 3 April.

They will now be joined from 2 to 6 May by 1,000 workers in non-examination roles in the same offices, and interview officers in Birmingham, Corby, Hemel Hempstead, Leeds, Portsmouth, Sheffield and Plymouth from 3 to 6 May.

PCS general secretary Mark Serwotka said the government had “insulted” his staff last week by announcing they will receive a pay rise this year of between 4.5% and 5%.

The 1,400 security officers at Heathrow who took part in 10 days of industrial action over the Easter period will now walk out on eight further days – 4-6 May, 9-10 May and 25-27 May.

Unite said the strikes will cause “inevitable disruption and delays” to passengers arriving for the King’s coronation because of “Heathrow’s stubborn refusal” to make an offer that meets members’ expectations.

Heathrow offered striking workers a 10% pay increase in January, but in talks last week it said it had improved the offer with a further £1,150 lump sum this year.

The airport said Unite was refusing to put the revised offer to its members.

“We kept Heathrow running smoothly during the first 10 days of Unite’s failed industrial action, and passengers can have confidence that we will do so again this time,” a Heathrow spokesperson said.

British Airways cancelled around 300 flights in advance due to the previous strike.

The FDA’s membership spans the civil service right up to permanent secretaries – the most senior rank of civil servant. It includes policy advisers, diplomats, lawyers, tax professionals, economists, statisticians and museum curators.

It is the first time the union has approved a national strike ballot over pay in more than 40 years.

It comes after the government’s decision last week to give civil servants a pay rise of between 4.5% and 5%.