MPs and peers are facing questions about hundreds of thousands of pounds a year in expenses for trips to meet foreign counterparts.
Politicians have run up big bills attending little-noted Council of Europe sessions in locations such as New York, Helsinki, Vienna, Madrid, Paris and Brussels.
Delegations to the parliamentary assemblies for Nato and the Organisation for Security and Co-operation in Europe (OSCE) have also spent significant sums.
In one case Liberal Democrat Mark Oaten is recorded as making a £2,000 taxpayer-funded trip to Athens for a meeting of a body that was being abolished - despite no longer being an MP.
Details of the expenditure have come to light following a Freedom of Information request by the Press Association.
The Parliamentary Assembly of the Council of Europe (Pace) meets four times a year to “debate current social problems and aspects of international politics”.
There are also 10 committees that prepare for the sessions.
The British delegation is appointed by the Prime Minister and reflects the composition of Parliament. There are 18 full members across both houses and another 18 “alternates” who can fill in when required.
According to material disclosed by the Commons, the delegation’s travel and subsistence costs came to £298,000 in 2013-14.
The bill for the first nine months of 2014-15 was more than £220,000.
The information reveals:
:: Mike Hancock represented the UK at a three-day meeting of the Political Affairs Committee in New York City in November last year – two months after he quit the Liberal Democrats over an “inappropriate” relationship with a vulnerable constituent.
His flights and travel came to £4,356, and hotel and subsistence £1,096.
:: Former deputy prime minister Lord Prescott went to Monaco for a one-day meeting about sustainable development in March 2011, with travel £750 and subsistence £574.
:: Tories Christopher Chope and Bob Neill had travel costs of £832 and £838 respectively for a two-day legal affairs committee meeting in Madrid last October.
Mr Neill said he had to “take what flights were available” at short notice due to business at Westminster, but they had been “within cost parameters” and booked by the Commons travel office.
He added that the meeting was “important” because there was a discussion of European Court of Human Rights (ECHR) decisions.
:: Hereditary peer the Earl of Dundee attended a one-day social affairs committee meeting in Paris in December last year, which cost £363 for travel and £320 for hotel and subsistence.
:: Jim Hood, Labour’s former MP for Lanark and Hamilton East, is listed as incurring a £1,400 travel bill to get to a session of the Migration, Refugees and Displaced Persons Committee in Paris in June 2013.
There was also a £305 hotel and subsistence bill for the one-day event.
The following May £1,190 was spent getting him to Athens for a meeting of the Political Affairs committee - and another £421 on subsistence.
:: The quarterly plenary sessions in Strasbourg last for five days and are typically attended by around 25 MPs and peers. The expenses bill for the British taxpayer from the event in October 2013 was around £48,000.
Information has also been released about the costs of delegations to the parliamentary assemblies for Nato and the OSCE.
The 257-strong Nato assembly is intended to “facilitate mutual understanding” and “promote debate on key security challenges”.
In 2013-14 – the most recent full year for which costs are available – the 18-member UK delegation incurred £235,666 in expenses.
Sir Hugh Bayley and Sir John Stanley went to South Korea last November for a four-day discussion about transatlantic defence and security cooperation.
Their travel bills were £5,026 and £4,954 respectively, and subsistence £1,677 and £1,506.
The OSCE assembly, meanwhile, features 320 delegates from 55 states, and carries out work including election monitoring and “developing mechanisms for conflict resolution”.
Expenses for the 13 British members came to £43,455 in 2013-14.
Sir Simon Burns observed the US elections on behalf of the OSCE for four days last November, with travel costing £4,056 and subsistence £930.
Sir Simon stressed all arrangements were made by the OSCE office.
Details have also been released by the Commons covering the European Security and Defence Assembly(ESDA), which was effectively abolished by the Lisbon Treaty.
Mr Oaten - who stood down at the general election in 2010 following a sex scandal and so ceased being an MP that April - is listed as attending a plenary session in Athens on May 10 and 11.
His travel cost £1,287 and subsistence £689.
A Commons spokesman said politicians could remain as delegation members for six months after an election, unless they are replaced earlier by the Prime Minister.
“The United Kingdom has been a leading member of the Council of Europe, Nato and OSCE since their inception,” the spokesman added. “The Parliamentary Assemblies of these bodies were set up to provide accountability to parliaments of member states and to assist and promote their work.
“The assemblies run a range of activities in pursuit of these aims including questioning ministerial representatives and debating sessions.”
TaxPayers’ Alliance chief executive Jonathan Isaby said: "While some of these trips may well be justified, transparency must be significantly improved so that taxpayers can judge whether or not each trip is truly worthwhile.
“Travel bills must be kept as low as possible, and many of the figures suggest that MPs were enjoying first- or business-class travel on the taxpayers' tab.
“In the midst of a necessary savings programme across government, spending on MPs' travel and overseas adventures cannot be immune."