MPs declared earnings of more than £7m from outside jobs and directorships over the last session of parliament.

An analysis by the Guardian shows 20 MPs made more money from their outside jobs than they did from their parliamentary salary of £65,738, with some spending more than 1,000 hours on outside employment.

Seventeen declared more than £100,000 in income during the 2012-13 parliamentary session, according to data filed on 7 May.

Gordon Brown earned more than £1.37m from giving speeches around the world. Brown said the income supported an office that he uses to "support my ongoing involvement in public life", with £600,000 going to charity and none of the money to him personally.

Under parliamentary rules, MPs are allowed to engage in outside work, including taking on company directorships, working as consultants, acting as barristers or simply giving speeches or writing books – provided that all income is declared in official registers, and no work relates to lobbying parliament.

Other top earners included Stephen Phillips, Conservative MP for Sleaford and North Hykeham, who declared more than £740,000 in outside income, generated by working more than 1,700 hours as a barrister, some of it on cases which began before the 2012-13 parliamentary session. Phillips said his outside work benefited his constituents by keeping a connection with the "real world".

"What matters is whether or not I do my job as an MP and how well I do it," he said. "People can check that from my attendance and other figures, which I believe demonstrate that I am one of the hardest workers at Westminster …

"The fact that I don't have a job as a minister and continue to work as a lawyer, mostly when parliament is not sitting and I am not engaged on constituency duties, enables me to keep a foot in the real world – though, I accept, a well paid one."

The high earners also included Geoffrey Cox, Tory MP for Torridge and West Devon, who declared £417,000 from acting as a barrister for clients including the government of Mauritius and Aegis Tax LLP. The former Conservative defence minister Nicholas Soames declared £305,000, from sources including work as a director of private security firm Aegis Defence Services and consultancy work for financial services companies.

Former Labour ministers Alistair Darling and Jack Straw declared earnings of £263,000 and £183,000 respectively. Straw said that his work as an MP allowed ample time for outside work, which was mainly a mixture of speaking engagements and writing.

"I devote around 60-70 hours to my duties as an MP, both national and constituency-related," he said. "After allowing for sleep, and family/social activities, there are another 30-40 hours available for my other work."

Conservative MPs declared more than £4.3m in earnings from outside directorships or jobs, against £2.4m for Labour – more than half of which was Brown's £1.37m. More than 50 MPs had directorships of at least one company, while 295 declared at least some kind of minimal earnings from outside parliament.

Transparency campaigners Unlock Democracy said the MPs with the largest outside earnings needed to reflect whether they were representing their constituents to the best of their abilities.

Alexandra Runswick, the lobby group's deputy director, said: "I doubt the majority of MPs can even imagine doing their jobs representing their constituents while raking in hundreds of thousands of pounds.

"The purpose of the allowances system is to allow MPs to work in parliament without having to be either independently wealthy, in employment or beholden to special interests; if MPs earn significant amounts outside parliament, at the very least the taxpayer should not be expected to subsidise them as well."

Runswick said she was in favour of MPs remaining able to do outside work but the findings highlighted the need for wider reform in Westminster.

Jonathan Isaby of the TaxPayers' Alliance said that while outside work was positive overall, voters should monitor their MPs to see it was not affecting their parliamentary work.

"Any other jobs [MPs] do should not distract them from their primary focus of serving their communities as MPs," he said.

The watchdog that has overseen MPs' pay and expenses since the 2009 expenses scandal, the Independent Parliamentary Standards Authority (Ipsa), is currently reviewing aspects of MPs' pay and other compensation, and its recent consultation included questions on the issue of outside work.

However, the Guardian understands that Ipsa will not recommend guidelines on second jobs when it publishes its proposals later this year. "Ipsa is currently reviewing MPs' pay and pensions and we will continue to consult the public as we do so," said Ipsa's head of communications, Mark Anderson. "This is the first time that MP remuneration will be set independently and not by MPs themselves."

Additional research: Mona Chalabi and Ami Sedghi.

• See the figures in full on the Guardian's Datablog.

• This article was amended on 28 May 2013 to reiterate in the picture caption that Gordon Brown has declared that he did not benefit personally from his outside earnings. He has declared in the register of members' interests that the earnings are "held by the Office of Gordon and Sarah Brown for the employment of staff to support my ongoing involvement in public life".