"With important presidential elections to come in the next few months, we must not allow the Taliban to disrupt the democratic process," Mr Brown said.
"Britain is therefore prepared to send additional troops as part of a wider Nato effort to strengthen security over the summer and until the elections as well as to ensure that our forces are properly protected from the growing threat of roadside bombs."
The prime minister denied that US President Barack Obama had asked directly for an extra 2,000 UK troops to be permanently deployed, but said the government had not ruled out sending extra forces in future.
He also said he had expressed his opposition to Mr Karzai about new measures covering Shia Muslims which campaigners say would legalise rape within marriage and restricts the freedom of women to go out without the their husbands' permission.
The issue "risks putting Afghanistan back to its past rather than towards a democratic future where men and women are treated equally", Mr Brown added.
"People will not accept that British soldiers are working in Afghanistan to make Afghanistan safe if the rights of women are not being properly upheld in the country. That is why I have demanded assurances from President Karzai," he said.
The president gave an assurance that he would make a statement on the subject and bring the law back before the Afghan Parliament, Mr Brown said.
The prime minister welcomed the appointment of Danish prime minister Anders Fogh Rasmussen as the next Nato secretary general, saying the decision by member states had been "unanimous".
But the decision to appoint Mr Rasmussen was attacked by Mohammed Shafiq, the leader of the UK Muslim group the Ramadhan Foundation.
Mr Rasmussen's candidacy had previously been fiercely opposed by Turkey after he defended freedom of speech during the controversy over the decision by a Danish newspaper to print cartoons of the prophet Mohammed.
NATO: KEY MOMENTS
Founded 1949, largely to block Soviet expansion into Europe
Twenty-six member states who vow to defend each other
Militarily dominated by the US
Acted in non-member state for first time in 1995 - implementing military aspects of Bosnia peace accord
Operated outside Europe for first time in 2003 - in Afghanistan
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